SCP can be scheduled multiple times for the same RS

SCP can be scheduled multiple times for the same RS:
1. a RS crashed, a SCP was submitted for it
2. before this SCP finish, the Master crashed
3. The new master will scan the meta table and find some region is still open on a dead server
4. The new master submit a SCP for the dead server again
The two SCP for the same RS can even execute concurrently if without HBASE-20846…

Provided a test case to reproduce this issue and a fix solution in the patch.

Another case that SCP might be scheduled multiple times for the same RS(with HBASE-20708.):
1. a RS crashed, a SCP was submitted for it
2. A new RS on the same host started, the old RS's Serveranme was remove from DeadServer.deadServers
3. after the SCP passed the Handle_RIT state, a UnassignProcedure need to send a close region operation to the crashed RS
4. The UnassignProcedure's dispatch failed since 'NoServerDispatchException'
5. Begin to expire the RS, but only find it not online and not in deadServer list, so a SCP was submitted for the same RS again