cbc-hash: allow to lookup servers by vbucket ID

Change-Id: Id0c097b5b2d10a4c765406f787aac39a2e13028a
Reviewed-on: http://review.couchbase.org/30089
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>