SSU: Rewrite adjustMTU()

to make it more responsive to conditions and a more effective PMTU algorithm.
Move MTU up and down in increments rather than simply shift between min and max
Increase or decrease based on success or failure of each message
Only increase if the successful message was almost as large as current MTU
Clear increase/decrease counters when changing MTU the other way
Don't wait for 10 packets to increase MTU
Reduce max retransmissions from 30% to 10%