[WFLY-1430] A clustering-monitor subsystem.

This pull request implements a subsystem called clustering-monitor, which among other things allows monitoring important clustering statistics on all nodes in a cluster from a single node. The clustering-subsystem is described in more detail in https://issues.jboss.org/browse/WFLY-1430.

This pull request replaces the initial pull request https://github.com/wildfly/wildfly/pull/4751.