BufferedPipe: the in-memory events needs to be loopback

We have 2 proxy threads, one in each direction. In the case of a real
interdomain connection they 'send' and 'recv' on the same event
channel number to both emit and wait for events. In the loopback
In_memory_events case, we need another thread to simulate being
the other domain, which will bounce the events back.

Signed-off-by: David Scott <dave.scott@citrix.com>