HBASE-24316 GCMulitpleMergedRegionsProcedure is not idempotent (#1660)

It addresses couple issues:
   1. Make sure deleteMergeQualifiers() does not delete the row if there is no columns with "merge" keyword.
   2. GCMulitpleMergedRegionsProcedure now acquire an exclusive lock on the child region.

Signed-off-by: stack <stack@apache.org>
3 files changed