Meta region assignment is blocked when all RS in meta table group are restarted.

This issue happened in a test cluster. The issue does not reproduce easily. But we can reproduce it with debug points in code.
Steps to reproduce:

1. Install a HBase cluster with three RS(rs1,rs2 and rs3) and one Master
2. Create two rsgroups r1 and r2 and move rs1 to r1 and rs2 to r2

   add_rsgroup 'r1';add_rsgroup 'r2';move_servers_rsgroup 'r1',['rs1Host:16020'];move_servers_rsgroup 'r2',['rs2Host:16020']
   

3. Create a table t1

   create 't1','f1','f2';put't1','r1','f1:c1','v1'

4. Start debugging master, put debug point in while loop of

   org.apache.hadoop.hbase.rsgroup.RSGroupInfoManagerImpl.ServerEventsListenerThread#run

   method.

5. Stop rs3
6. When debug flow comes, wait around 30 seconds to let the meta be offline and then let the debug flow execute. By now meta will be offline as rs3 is stopped. HMaster UI will hang as meta is offline.
7. Now start rs3, after start meta should be online and Master UI should open.
8. No, still master UI hangs, then you have reproduced the issue.