Split procedure rollback can leave parent region state in SPLITTING after completion

Depending on where the split procedure fails, SplitTableRegionProcedure rollback can leave the parent region's in memory RegionStateNode in SPLITTING after rollback is complete, when the parent region is still online on the assigned region server and according to meta. This leaves active HMaster believing that the parent region is offline according to its RegionStates, and causes subsequent procedures that require the region to be online like merge/split/move to fail to start. One workaround is to restart active HMaster to reset the in memory record of region states. 

Two example scenarios where this can happen:

• If we get to SPLIT_TABLE_REGION_CLOSE_PARENT_REGION and the parent region has a replica which is in transition, the unassign procedure in that step is never created/rolled back and we are left with the parent region state in splitting.
• If region quotas are enabled and a split is run for a region whose namespace is at its maximum region quota limit we will fail in SPLIT_TABLE_REGION_PRE_OPERATION with QuotaExceededException and we are left with the parent region state in splitting

The region replica case is demonstrated in TestSplitTableRegionProcedure.testRollbackForSplitTableRegionWithReplica.

To reproduce the region quota case in HBase shell:

> create_namespace 'test_ns', {'hbase.namespace.quota.maxregions'=> 2}
> create 'test_ns:test_table', 'f1', {NUMREGIONS => 2, SPLITALGO => 'UniformSplit'}
> region_a = <first region from list_regions 'test_ns:test_table'>
> region_b = <second region from list_regions 'test_ns:test_table'>

> split region_a, 'x'
# HMaster will report: 
pid=405, state=ROLLEDBACK, exception=org.apache.hadoop.hbase.quotas.QuotaExceededException via master-split-regions:org.apache.hadoop.hbase.quotas.QuotaExceededException: Region split not possible for :<region_a> as quota limits are exceeded ; SplitTableRegionProcedure table=test_ns:test_table, parent=...

> merge_region region_a, region_b
ERROR: org.apache.hadoop.hbase.exceptions.MergeRegionException: org.apache.hadoop.hbase.client.DoNotRetryRegionException: <region_a> is not OPEN; state=SPLITTING

> stop_master # trigger hmaster failover 
> merge_region region_a, region_b # merge now succeeds