Allow connector to not define maximum capacity (e.g. Undertow listener)

See WFLY-1888.

In WF we are wrapping Undertow listener with Connector interface. However, with all the async processing the maximum capacity does not represent busyness correctly.

I am going to add support for the connector to return "-1" that indicates to mod_cluster that this connector does not define maximum number of threads and leave the load calculation to capacity instead.