JCR-3821 SecureRandom#generateSeed may take a (very) long time to return. To prevent the SeededSecureRandom from stalling shutdown, call it on a daemon thread.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/trunk@1631995 13f79535-47bb-0310-9956-ffa450edef68