A worker thread class which can drain channels and echo-back the input. Each
instance is constructed with a reference to the owning thread pool object.
When started, the thread loops forever waiting to be awakened to service the
channel associated with a SelectionKey object. The worker is tasked by
calling its serviceChannel() method with a SelectionKey object. The
serviceChannel() method stores the key reference in the thread object then
calls notify() to wake it up. When the channel has been drained, the worker
thread returns itself to its parent pool.
The actual code which drains the channel associated with
the given key. This method assumes the key has been
modified prior to invocation to turn off selection
interest in OP_READ. When this method completes it
re-enables OP_READ and calls wakeup() on the selector
so the selector will resume watching this channel.