BUG-3219: implement a shared stash for queue reuse

BUG-3219: implement a shared stash for queue reuse

This introduces a global per-size stash for reuse of OutbountQueueImpl
objects. This allows for efficient reuse across multiple sessions, which
means we minimize stale memory use when some channels are idle.

The stash is implemented in terms of a ConcurrentLinkedDeque, which is
flushed from the tail and has soft references. We rely on
FinalizableReference to notify us when a particular queue is finished.

Change-Id: I78e654a0c3470bbd1c9274237b2be04f50459809
Signed-off-by: Robert Varga <rovarga@cisco.com>