new scheme for managing port deletion

new scheme for managing port deletion

shared_ptr<Port> now uses a deleter functor which pushes Port* to a lock-free FIFO so that the Port is
always deleted (and thus unregistered with the PortEngine/backend) in a safe context w.r.t. various
callbacks in the host. Currently the auto_connect_thread in Session has been tasked with doing these
deletions.