The stub in AsyncConnectionImpl.rsStubs and adminSubs never gets cleaned up

Currently there is no way for us to remove the unused stub in these two maps, and after HBASE-25292, things get worse as we will use ServerName directly as the stub key which means every time a region server restarts, we will have a new entry in the map.

In the past, since the name only contains host and port(and ip address if hostnameCanChange is true), so we will not have too many entris in these two maps unless we replaced the current region servers with new ones, but it could still be limited to the machines we have.

So here I propose that we just use host and port directly as now we will do a resolve when connecting, to get the old behavior back.

And maybe we should also find a way to cleanup these two maps.