BUG-8619: do not touch forward path during purge enqueue

BUG-8619: do not touch forward path during purge enqueue

In case of a purge request, the request is sent from the head
of a connection chain (i.e. the original connection which created
the transaction) and propagated via forwarders. This path needs
to make sure it does not go via throttling, as it is an internal
detail.

Separate the transmit paths a bit more, so that TransmitQueue
can push messages to forwarders' replay path.

Change-Id: I5e146b8d11e8654b4beae3959207efb9c2f18315
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>