MB-7435: Add a command to get estimates of keys to move

Change-Id: I8c7dfd23dceed93c8533891128c6448b284aa02f
Reviewed-on: http://review.couchbase.org/25244
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>