Move PDU parsing to RTR server threads

Move PDU parsing to RTR server threads

PDU parsing has been done by the poller up to this point.
I probably intended this to encapsulate all the stream operations in
the poller.

During 4e0f81dd5405f8c6c48d2e74c0468d16b44f9a50, the poller's block of
code that contained PDU parsing had to be moved to the mutex time.
This created a substantial incentive to migrate PDU parsing out of the
poller.

So now it's done by the RTR server threads instead.