CCBC-502: Provide lcb_cntl API to retrieve bucket name

Change-Id: I85f34ca43b5b8563795de8f1142d0744aa43cf83
Reviewed-on: http://review.couchbase.org/40987
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>