Guard against double channel close

[onitake]

Fixes: #34

Closure of the output queue channel is guarded against by adding an atomic flag around it.
To make the code more readable and slightly less error-prone, the output queue is now fully abstracted, and its internal state can no longer be accessed directly.

Note that this does not prevent a potential race condition: If writing into the queue concurrently with a close(), the runtime may panic. It's vital that Close() is never executed while a write may be in progress. The PR addresses that by removing the connection from the pool directly after close (when entering inhibit) and by waiting for completion of the remove from pool command when the outgoing connection is closed.