CCBC-303: Allow non-2^n vbuckets

Change-Id: I6df69ad1e525a7d07e73841a997789422122880a
Reviewed-on: http://review.couchbase.org/32330
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>