Change default Bitcask I/O mode to Erlang

Additional testing demonstrates that using Erlang's built-in I/O provides more consistent behavior than the synchronous NIF approach. While the NIF approach is known to provide higher throughput, the non-deterministic impact on the Erlang VM from the use of blocking NIFs outweighs the benefit. Defaulting to the safer option. Users can manually revert to using NIFs if desired.