MCMPHandler#handleRequest() does not work with IPv6 addresses

This is continuation of the JBEAP-1767 saga.

The code at
https://github.com/undertow-io/undertow/blob/1.3.18.Final/core/src/main/java/io/undertow/server/handlers/proxy/mod_cluster/MCMPHandler.java#L146
will not match IPv6 addresses properly.