MB-10380: Implementation of get_num_items/2

With `couch_upr_client:get_num_items/2` it is possible to receive
the number of items a vBucket/partition contains directly from
ep-engine (instead of getting it from the database files).

Change-Id: I397a6dd177075c47a29da72c83d593b188fe2032
Reviewed-on: http://review.couchbase.org/34438
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>