DERBY-6631: FileMonitor can be used to elevate an application's privileges

Don't elevate privileges in the setThreadPriority() method. Instead, let
the callers use doPrivileges() themselves.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1620534 13f79535-47bb-0310-9956-ffa450edef68