HBASE-22941 merge operation returns parent regions in random order (#556)

HBASE-22941 merge operation returns parent regions in random order (#556)

* HBASE-22941 merge operation returns parent regions in random order

store and return the merge parent regions in ascending order

remove left over check for exactly two merged regions

add unit test

* use SortedMap type to emphasise that the Map is sorted.

* use regionCount consistently and checkstyle fixes

* Delete tests that expect multiregion merges to fail.

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