Throttle RPC and WS concurrent active clients

This change set implements tuneable concurrent active clients throttling.
btcd uses the following:
```
defaultMaxRPCClients = 10
defaultMaxRPCWebsockets = 25
```

I'm not sure those are the best defaults, but it probably makes sense to be consistent since wallet passes through many requests to btcd.  If wallet supports more than btcd by default, and btcd rejects some requests while wallet allows them, it might not be obvious what is going on.