EsbDeployment stop/start mbean functions do not restart listeners

The EsbDeployment registers as an MBean within the jmx-console, with name jboss.esb:deployment=<name>, and exposes a start and stop method.

Stopping the deployment results in the listeners being stopped, however starting the deployment again does nothing.

The simple solution is to null the reference to the controller when stopping, adding a guard for null within that method.

We should also add a state so that this can be queried.