Better handling of dfsadmin command when namenode is slow

Probably when hitting HADOOP-3810, Namenode became unresponsive. Large time spent in GC.
All dfs/dfsadmin command were timing out.

WebUI was coming up after waiting for a long time.

Maybe setting a long timeout would have made the dfsadmin command go through.
But it would be nice to have a separate queue/handler which doesn't compete with regular rpc calls.

All I wanted to do was dfsadmin -safemode enter, dfsadmin -finalizeUpgrade ...