[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 9d8b107)
(cherry picked from commit 276363b41460193390d311ae86078dc80b7ee196)