[BZ 1076258] Fix classloader leak in as7 plugin

The plugin-wide thread pool for closing stale connections was leaving
threads running and those threads held on to the "old" plugin classloaders
from previous plugin container "runs".

The AS7 plugin now has a plugin lifecycle listener that shuts down that
thread pool preventing that leak from happening.

(cherry picked from commit 9d8b107414e926361ff2b7501e2094402de8e7f7)
Signed-off-by: Libor Zoubek <lzoubek@redhat.com>

Conflicts:
	modules/plugins/jboss-as-7/src/main/java/org/rhq/modules/plugins/jbossas7/ASConnection.java