Remove unused lcb_t members: vb_server_map and nvbuckets

It should save us some memory per each lcb_t

Change-Id: Idf625f5d50a6ebeff105dd00d7872cb772316099
Reviewed-on: http://review.couchbase.org/30406
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>