Fix: channel management thread should hold a refcount

In order to handle teardown caused by relayd being unresponsive, we need
to ensure that we correctly handle that the streams can be cleaned up
before the channel wakeup pipe gets closed. We achieve this by making
the channel management thread hold a refcount on the channel object, so
it only gets destroyed when all streams, _and_ the channel management
thread, have released their reference.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>