CCBC-318: Implement key stats feature

This adds a new command options to the lcb_CMDSTATS structure which
allows to interpret the key as a document ID for which stats should be
returned from its related nodes.

Change-Id: Ic5a93dba109a5973cc64b7c1c0cff1e87cb77ac5
Reviewed-on: http://review.couchbase.org/41288
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>