HyperSQL Database Engine

Upon shutdown, the HsqlTimer.TaskRunner needs to nullify the thread
"taskRunnerThread" contextClassLoader property via a
setContextClassloader(null) on shutdown()

By not doing this, if hsql is used in a hot-deployed environment, such as
in Tomcat. When the context is destroyed and if shutdown() is called on the
database, this thread retains a reference to the WebappClassloader, which
will never be cleaned up out of the heap because a JVM thread group retains
a reference to this thread. Over time this will lead to a permgen out of
memory exception

[jvm thread group] -> [hsqltimer.taskRunner thread].contextClassLoader ->
WebappClassloader -> [entire application]