Rework xgress goroutine model

[plorenz]

1. Allow pull model to get rid of txer goroutine
2. Try moving retransmission sends into link send buffer. That way it can block and avoid drops. Alternately, move back to rtxer per xgress
3. Try moving back to payload ingester go-routine per xgress. Or maybe we don't need it at all, since we have locking now.