relay: fix potential thread hang on exit

To make sure there is no status message pending in the tx pipe,
relay_cbus_detach() waits on relay->status_cond before proceeding to
relay destruction. The status_cond is signaled by the status message
completion routine (relay_status_update()) handled by cbus on the
relay's side. The problem is by the time we call relay_cbus_detach(),
the cbus loop has been stopped (see relay_subscribe_f()), i.e. there's
no one to process the message that is supposed to signal status_cond.
That means, if there happens to be a status message en route when the
relay is stopped, the relay thread will hang forever.

To fix this issue, let's introduce a new helper function, cbus_flush(),
which blocks the caller until all cbus messages queued on a pipe have
been processed, and use it in relay_cbus_detach() to wait for in-flight
status messages to complete. Apart from source and destination pipes,
this new function takes a callback to be used for processing incoming
cbus messages, so it can be used even if the loop that is supposed to
invoke cbus_process() stopped.