avoid deadlock in AMQPUrlReceiver hopefully
use "fair" ReentrantLock -- I stepped through heritrix that appeared to be deadlocked, found that the thread trying to shut down was being starved of the lock by another thread (the StarterRestarter) locking and unlocking it repeatedly, which can apparently happen because the lock was "unfair"
Merge pull request #102 from nlevitt/amqp-receiver-deadlock

avoid deadlock in AMQPUrlReceiver hopefully