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.