tcp: shift PRR sending cadence slightly left

Don't let PRR pass up on the opportunity of clocking
out packets on arrival of ACKs - by pulling sends
forward by about half a packet. Prevents unexpectedly
long runs of incoming ACKs without eliciting a
packet transmission.

MFC after: 1 week
Reviewed By: transport, tuexen
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D42918