MULE-8013: Added connections configuration parameters for incoming and o...

...utgoing requests.
This has been discussed elsewhere i know, but IMO "concurrent" here is obvisouly implicit.  It's very normal for http servers/clients to configure maxConnections without 'concurrent' explicit.  Using "concurrent" here would mean we should use "maxConcurrentPersistentConnections" for <listener> for consistency, but this is a horrible mouthful.