Adds region state check on hbck2 assigns/unassigns. Returns pid of -1
if in inappropriate state with logging explaination which suggests
passing override if operator wants to assign/unassign anyways. Here
is an example of what happens now if hbck2 tries an unassign and
Region already unassigned:

  2020-08-19 11:22:06,926 INFO  [RpcServer.default.FPBQ.Fifo.handler=1,queue=0,port=50086] assignment.AssignmentManager(820): Failed {ENCODED => d1112e553991e938b6852f87774c91ee, NAME => 'TestHbck,zzzzz,1597861310769.d1112e553991e938b6852f87774c91ee.', STARTKEY => 'zzzzz', ENDKEY => ''} unassign, override=false; set override to by-pass state checks.
  org.apache.hadoop.hbase.client.DoNotRetryRegionException: Unexpected state for state=CLOSED, location=null, table=TestHbck, region=d1112e553991e938b6852f87774c91ee
          at org.apache.hadoop.hbase.master.assignment.AssignmentManager.preTransitCheck(AssignmentManager.java:583)
          at org.apache.hadoop.hbase.master.assignment.AssignmentManager.createOneUnassignProcedure(AssignmentManager.java:812)
          at org.apache.hadoop.hbase.master.MasterRpcServices.unassigns(MasterRpcServices.java:2616)
          at org.apache.hadoop.hbase.shaded.protobuf.generated.MasterProtos$HbckService$2.callBlockingMethod(MasterProtos.java)
          at org.apache.hadoop.hbase.ipc.RpcServer.call(RpcServer.java:397)
          at org.apache.hadoop.hbase.ipc.CallRunner.run(CallRunner.java:133)
          at org.apache.hadoop.hbase.ipc.RpcExecutor$Handler.run(RpcExecutor.java:338)
          at org.apache.hadoop.hbase.ipc.RpcExecutor$Handler.run(RpcExecutor.java:318)

Previous it would just create the unassign anyways. Now must pass override
to queue the procedure regardless. Safer.

hbase-server/src/main/java/org/apache/hadoop/hbase/master/MasterRpcServices.java
 javadoc on assigns/unassigns. Minor refactor in assigns/unassigns to cater to
 case where procedure may come back null (if override not set and fails state checks).

hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/AssignmentManager.java
 checkstyle cleanups.
 Clarifying javadoc on how there is no state checking when bulk assigns creating/enabling
 tables.

 createOneAssignProcedure and createOneUnassignProcedure now handle exceptions which now
 can be thrown if no override and region state is not appropriate.

 Aggregation of createAssignProcedure and createUnassignProcedure instances adding in
 region state check invoked if override is NOT set.

hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/RegionStateNode.java
 Change to setProcedure so it returns passed proc as result instead of void

Signed-off-by: Duo Zhang <zhangduo@apache.org>
(cherry picked from commit c7e31f7)