Flooding the cluster with administrative actions leads to collapse

Steps to reproduce:
1. Start a cluster.
2. Start an ingest process.
3. In the HBase shell, do this:

while true do
   flush 'table'
end

We should reject abuse via administrative requests like this.

What happens on the cluster is the requests back up, leading to lots of these:

2014-01-10 18:55:55,293 WARN  [Priority.RpcServer.handler=2,port=8120] monitoring.TaskMonitor: Too many actions in action monitor! Purging some.

At this point we could lower a gate on further requests for actions until the backlog clears.

Continuing, all of the regionservers will eventually die with a StackOverflowError of unknown origin because, stack overflow:

2014-01-10 19:02:02,783 ERROR [Priority.RpcServer.handler=3,port=8120] ipc.RpcServer: Unexpected throwable object java.lang.StackOverflowError
        at java.util.ArrayList$SubList.add(ArrayList.java:965)
[...]