Support backing up to an instance of sqlite3.Database

[SmartArray]

This PR introduces the ability to use an instance of sqlite3.Database to perform backups using the Online Backup API.

This PR is a work in progress until yarn test runs through successfully.

[SmartArray]

Help needed

The call to the backup API is causing a segfault, currently.

I suppose this line is causing the issue: https://github.com/SmartArray/node-sqlite3/blob/401937943af31cd6e7844deb124e315df4c58400/src/backup.cc#L161

I could not figure out why the destructor of the Backup instance causes this bad memory access (a double free)?

I would love some assistance in this issue! Unfortunately I have no experience using the N-API.

Here is the backtrace I gathered with LLDB:

* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0xbeaddccdaa08)
  * frame #0: 0x0000000100078e20 node`napi_delete_reference + 52
    frame #1: 0x0000000107cef270 node_sqlite3.node`Napi::Reference<Napi::Object>::~Reference() + 76
    frame #2: 0x0000000107ceba18 node_sqlite3.node`Napi::ObjectWrap<node_sqlite3::Backup>::~ObjectWrap() + 252
    frame #3: 0x0000000107cfba08 node_sqlite3.node`node_sqlite3::Backup::~Backup() + 196
    frame #4: 0x0000000107ceeb28 node_sqlite3.node`node_sqlite3::Backup::~Backup() + 28
    frame #5: 0x0000000107ceeb54 node_sqlite3.node`node_sqlite3::Backup::~Backup() + 28
    frame #6: 0x0000000107cf53dc node_sqlite3.node`Napi::ObjectWrap<node_sqlite3::Backup>::ConstructorCallbackWrapper(napi_env__*, napi_callback_info__*)::'lambda'()::operator()() const + 212
    frame #7: 0x0000000107cf52e8 node_sqlite3.node`napi_value__* Napi::details::WrapCallback<Napi::ObjectWrap<node_sqlite3::Backup>::ConstructorCallbackWrapper(napi_env__*, napi_callback_info__*)::'lambda'()>(Napi::ObjectWrap<node_sqlite3::Backup>::ConstructorCallbackWrapper(napi_env__*, napi_callback_info__*)::'lambda'()) + 32
    frame #8: 0x0000000107cf3eec node_sqlite3.node`Napi::ObjectWrap<node_sqlite3::Backup>::ConstructorCallbackWrapper(napi_env__*, napi_callback_info__*) + 148
    frame #9: 0x000000010007b36c node`v8impl::(anonymous namespace)::FunctionCallbackWrapper::Invoke(v8::FunctionCallbackInfo<v8::Value> const&) + 108
    frame #10: 0x0000000100270a38 node`v8::internal::FunctionCallbackArguments::Call(v8::internal::CallHandlerInfo) + 276
    frame #11: 0x000000010027026c node`v8::internal::MaybeHandle<v8::internal::Object> v8::internal::(anonymous namespace)::HandleApiCallHelper<true>(v8::internal::Isolate*, v8::internal::Handle<v8::internal::HeapObject>, v8::internal::Handle<v8::internal::HeapObject>, v8::internal::Handle<v8::internal::FunctionTemplateInfo>, v8::internal::Handle<v8::internal::Object>, v8::internal::BuiltinArguments) + 504
    frame #12: 0x000000010026fd9c node`v8::internal::Builtin_HandleApiCall(int, unsigned long*, v8::internal::Isolate*) + 196
    frame #13: 0x00000001009b088c node`Builtins_CEntry_Return1_DontSaveFPRegs_ArgvOnStack_BuiltinExit + 108
    frame #14: 0x0000000100941610 node`Builtins_JSBuiltinsConstructStub + 368
    frame #15: 0x000000011010e934
    frame #16: 0x00000001009443d8 node`Builtins_InterpreterEntryTrampoline + 248
    frame #17: 0x00000001009421cc node`Builtins_JSEntryTrampoline + 172
    frame #18: 0x0000000100941e64 node`Builtins_JSEntry + 164
    frame #19: 0x0000000100313a24 node`v8::internal::(anonymous namespace)::Invoke(v8::internal::Isolate*, v8::internal::(anonymous namespace)::InvokeParams const&) + 2388
    frame #20: 0x00000001003130b8 node`v8::internal::Execution::Call(v8::internal::Isolate*, v8::internal::Handle<v8::internal::Object>, v8::internal::Handle<v8::internal::Object>, int, v8::internal::Handle<v8::internal::Object>*) + 200
    frame #21: 0x0000000100220548 node`v8::Function::Call(v8::Local<v8::Context>, v8::Local<v8::Value>, int, v8::Local<v8::Value>*) + 520
    frame #22: 0x0000000100076924 node`napi_call_function + 168
    frame #23: 0x0000000107cefecc node_sqlite3.node`Napi::Function::Call(napi_value__*, unsigned long, napi_value__* const*) const + 64
    frame #24: 0x0000000107ce9c10 node_sqlite3.node`Napi::Function::Call(napi_value__*, std::__1::vector<napi_value__*, std::__1::allocator<napi_value__*> > const&) const + 80
    frame #25: 0x0000000107d00618 node_sqlite3.node`node_sqlite3::Database::Work_AfterOpen(napi_env__*, napi_status, void*) + 1468
    frame #26: 0x000000010008af98 node`(anonymous namespace)::uvimpl::Work::AfterThreadPoolWork(int) + 136
    frame #27: 0x0000000100922764 node`uv__work_done + 192
    frame #28: 0x0000000100925f00 node`uv__async_io + 320
    frame #29: 0x0000000100937c78 node`uv__io_poll + 1036
    frame #30: 0x0000000100926390 node`uv_run + 380
    frame #31: 0x0000000100002ccc node`node::SpinEventLoop(node::Environment*) + 244
    frame #32: 0x00000001000ed6e0 node`node::NodeMainInstance::Run(int*, node::Environment*) + 120
    frame #33: 0x00000001000ed3ac node`node::NodeMainInstance::Run(node::EnvSerializeInfo const*) + 120
    frame #34: 0x00000001000872e0 node`node::Start(int, char**) + 184
    frame #35: 0x000000010499d08c dyld`start + 520

[SmartArray]

Fixed all memory issues, tests and added typescript declarations.
Integrated into production environment of a project. Seems to work well.

[SmartArray]

I am not sure if the Ref() call is needed here. I have applied this from @paulfitz.
I assume it is needed because it prevents the otherDb instance to be freed while a backup is in progress, which should usually not happen unless the user closes the db during a backup process.

Please someone double check.