MB-7966: Change VBucketMap name from vbuckets to vbMap

Change-Id: I215871766f95142bde8a866d8a52d7d549d99def
Reviewed-on: http://review.couchbase.org/25692
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>