Each bucket can have 1 or more bucketstores.

When a vbucket wants to persist, it'll persistet to one
of the bucketstores by modulus.

This is a stepping stone in the move away from 1-file-per-vbucket to
N-files-per-bucket, where 1 or more vbuckets might share the same
storage file.
Renamed to DEFAULT_BUCKET_NAME for consistency.
Parametrized default bucket name more.