pool.js and pool_connection.js required index.js, and
index.js required pool.js and pool_connection.js.

Circular dependency can cause all sorts of problems.
This fix aims to provide a step towards fixing a problem with
@opentelemetry/instrumentation-mysql2, which looses several
exports when using its patched require.

For instance, `format` is lost and can cause an instrumented
application to crash.

index.js and promise.js require each other.

Circular dependency can cause all sorts of problems.
This commit aims to fix a problem with
@opentelemetry/instrumentation-mysql2, which looses several
exports when using its patched require.

For instance, `format` is lost and can cause an instrumented
application to crash.

The proposal to put common.js into lib was good, but it felt
weird to arbitrarily stuff just some exported functions in
lib/common.js. This made sense when common.js was meant
to provide commons for index.js and promise.js. But
in the lib, this felt like a weird scope.

I therefore split common.js into

- lib/create_connection.js
- lib/create_pool.js
- lib/create_pool_cluster.js

Also made `require` more consistent in all affected files: all
`require` files now have a js suffix when they refer to a
single local file.

promise.js was required by lib sources, and promise.js required
the same lib sources eventually. Extracted the respective
classes to lib. Also extracted functions that are
shared between several of the new files.

Decided to put each exported class / function into its own file.
This may bloat the lib folder a bit, but it provides clarity
(where to find what).

The extraction of classes was performed without extracting
the patch of methods like `escape`, etc.
The patching is now performed in the files that define the
respective classes, guaranteeing patched versions when
these files are required.

Co-authored-by: mknj <[email protected]>