DERBY-6619 After silently swallowing SecurityExceptions, Derby can leak class loaders

Patch derby-6619 which prints warnings on derby.log if permissions are
missing to get and set the context class loader in
SingletonTimerFactory (which can lead to classloader leak). Also adds
a test of this by checking derby.log in SecureServerTest.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1618955 13f79535-47bb-0310-9956-ffa450edef68