distributedLogReplay may hang during chained region server failure

In a test, half cluster(in terms of region servers) was down and some log replay had incurred chained RS failures(receiving RS of a log replay failed again).

Since by default, we only allow 3 concurrent SSH handlers(controlled by

this.executorService.startExecutorService(ExecutorType.MASTER_SERVER_OPERATIONS,conf.getInt("hbase.master.executor.serverops.threads", 3));

).

If all 3 SSH handlers are doing logReplay(blocking call) and one of receiving RS fails again then logReplay will hang because regions of the newly failed RS can't be re-assigned to another live RS(no ssh handler will be processed due to max threads setting) and existing log replay will keep routing replay traffic to the dead RS.

The fix is to submit logReplay work into a separate type of executor queue in order not to block SSH region assignment so that logReplay can route traffic to a live RS after retries and move forward.