Add rpcinfo rpc to debug deadlocks

We seem to have deadlock bugs in our RPC system, most likely inherited from ZEC or BTC.
Since some Hush RPC's take longer (such as anything with Sietch protections), the deadlocks
are more likely to occur. Eventually all RPC slots are used up and no more RPC commands
can be sent to hushd. This is why the "plz_stop" feature was implemented, but that is just
a workaround to restart the server. We must find and fix the root cause.

This rpc will allow us to see when we are getting close to our maximum work queue depth
and hopefully help us learn exactly what is happening.

src/httpserver.h

@@ -24,6 +24,9 @@ struct event_base;
 class CService;
 class HTTPRequest;
 
+int getWorkQueueDepth();
+int getWorkQueueMaxDepth();
+
 /** Initialize HTTP server.
  * Call this before RegisterHTTPHandler or EventBase().
  */