Implement Copy on Write

[itsmontoya]

goos: linux
goarch: amd64
pkg: github.com/itsmontoya/whiskey
BenchmarkWhiskeyGet-16              2000            847693 ns/op          176011 B/op       7000 allocs/op
BenchmarkWhiskeyPut-16               100          12207011 ns/op          720110 B/op      25000 allocs/op
BenchmarkWhiskeyBatchPut-16        10000            167264 ns/op             721 B/op         25 allocs/op
BenchmarkBoltGet-16                 1000           1553200 ns/op          488010 B/op       8000 allocs/op
BenchmarkBoltPut-16                    1        2458374706 ns/op        34075920 B/op      49951 allocs/op
BenchmarkBoltBatchPut-16             300           3770269 ns/op          161626 B/op       3103 allocs/op
BenchmarkLMDBGet-16                 1000           1400146 ns/op          120056 B/op       5001 allocs/op
BenchmarkLMDBPut-16                    1        2340215682 ns/op          124312 B/op       4025 allocs/op
BenchmarkLMDBBatchPut-16             500           3230487 ns/op             132 B/op          4 allocs/op