BUGFIX: base ACK timeout on ACK TX, not packet RX

This effectively undoes (most of) commit 6b8a8b659a3d33

The original behaviour makes more sense... When you send
an ACK to the sender, you want to wait Timeout ticks
before assuming something's gone missing and retransmit
an ACK. If we base the timeout on the last packet receive
time, this gives the sender less time to respond, which
did nog make a whole lot of sense.