[AMv2] Split/Merge need cleanup; currently they diverge and do not fully embrace AMv2 world

Region Split and Merge work on the new AMv2 but they work differently. This issue is about bringing them back together and fully embracing the AMv2 program.

They both have issues mostly the fact that they carry around baggage no longer necessary in the new world of assignment.

Here are some of the items:

Split and Merge metadata modifications are done by the Master now but we have vestige of Split/Merge on RS still; e.g. when we SPLIT, we ask the Master which asks the RS, which turns around, and asks the Master to run the operation. Fun. MERGE is all done Master-side.

Clean this up. Remove asking RS to run SPLIT and remove RegionMergeRequest, etc. on RS-side. Also remove PONR. We don’t Points-Of-No-Return now we are up on Pv2. Remove all calls in Interfaces; they are unused. Make RS still able to detect when split, but have it be a client of Master like anyone else.

Split is Async but does not return procId

Split is async. Doesn’t return the procId though. Merge does. Fix. Only hard part here I think is the Admin API does not allow procid return.

Flags

Currently OFFLINE is determined by looking either at the master instance of HTD (isOffline) and/or at the RegionState#state. Ditto for SPLIT. For MERGE, we rely on RegionState#state. Related is a note above on how split works – there is a split flag in HTD when there should not be.

TODO is move to rely on RegionState#state exclusively in Master.

From Split/Merge Procedures need finishing in https://docs.google.com/document/d/1eVKa7FHdeoJ1-9o8yZcOTAQbv0u0bblBlCCzVSIn69g/edit#heading=h.4b60dc1h4m1f