JCBC-513: Add support for individual bucket close.

Change-Id: I1f50f14bf054d6ec468b1ed88f7ad6ce28a45060
Reviewed-on: http://review.couchbase.org/40827
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>