Auto-rebalancer: clear replace marker on failed moves

When CheckMoveCompleted fails, best-effort clear the replace marker on
the source replica and drop the move so rebalancing can continue.
Add a test that forces move failure and verifies no replica remains
marked for replacement.

Change-Id: I99dafa654878b9d6d8162d84500913ae0655692b
Reviewed-on: http://gerrit.cloudera.org:8080/21073
Reviewed-by: Zoltan Chovan <zchovan@cloudera.com>
Tested-by: Zoltan Chovan <zchovan@cloudera.com>
Reviewed-by: Marton Greber <greber.mx@gmail.com>