[AMv2] expireServers and ServerCrashProcedure cleanup

In review of HBASE-18551, uagashe posed a scenario that revealed a hole in our processing of unassigns; there is case where a UP might not get notification from ServerCrashProcedure if the UP is scheduled AFTER a SCP has gotten past its handleRIT call (No new SCP will be queued because expireServer won't let it happen if crashed server is in dead server list which it will be).

Chatting on it, expireServers is doing checks that belong inside ServerCrashProcedure. expireServers scheduling an SCP each time it is called would make it so SCP processing is serialized one behind the other. If the first does the clean up all subsequent will do no work but Procedures dependent on them will get their wakeup call.

This issue is about implementing the above cleanup.