ACCUMULO-2491 Correct table user state upon termination of Security randomwalk

A partial backport of ACCUMULO-1162 to 1.4 - commit e93a11a only. As the Security randomwalk
ends and drops the table user, it now also sets the user's existence flag to false.
Merge branch '1.4.5-SNAPSHOT' into 1.5.2-SNAPSHOT (-sours)