Disable region splits and merges switch in master

In large clusters where region splits are frequent, and HBCK runs take longer, the concurrent splits cause further problems in HBCK since HBCK assumes a static state for the region partition map. We have just seen a case where HBCK undo's a concurrently splitting region causing number of inconsistencies to go up.

We can have a mode in master where splits and merges are disabled like the balancer and catalog janitor switches. Master will reject the split requests if regionservers decide to split. This switch can be turned on / off by the admins and also automatically by HBCK while it is running (similar to balancer switch being disabled by HBCK).

HBCK should also disable the Catalog Janitor just in case.