ISPN-2962 Fix thread leaks in the core test suite

Several tests do not shut down their Executors or fail to stop an AsyncStore leading to stray threads after the suite is run.

This only partially fixes https://issues.jboss.org/browse/ISPN-2477 , please leave it open.

Please integrate in master.

JIRA: https://issues.jboss.org/browse/ISPN-2962