memrowset: skip all uncommitted mutations when iterating
memrowset: skip all uncommitted mutations when iterating

We already take advantage of this optimization the DMS and DeltaFiles; not
sure why we weren't doing it in the MRS.

Change-Id: I6f4049a1524e04660d853474a0c140c602c88d33
Reviewed-on: http://gerrit.cloudera.org:8080/13531
Tested-by: Kudu Jenkins
Reviewed-by: Grant Henke <4cf7ebbe638391c4d27a10cf751b99bdbd1a1880@apache.org>
Reviewed-by: Mike Percy <e07a1fd797b473a9c849e618aa2ce06d8055760b@apache.org>

helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu