MB-8729: Add get_engine_vb_map interface for CCCP

Try to call back into the engine to get the current vbucket
map if the engine returns NOT_MY_VBUCKET

Change-Id: I2ae2e140e6b8f9f03d8e3721240d178ceb51a905
Reviewed-on: http://review.couchbase.org/29844
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>