Send one MTU probe at a time.

Send one MTU probe at a time.

Currently, tinc sends MTU probes in batches of three every second. This
commit changes that to send one packet every 333 milliseconds instead.

This change brings two benefits:

 - It makes MTU probing faster, because MTU probe lengths are calculated
   based on minmtu, and minmtu is adjusted based on the replies. When
   sending batches of three packets, all three packets are based on the
   same minmtu estimation; in contrast, by sending one packet more
   frequently, each subsequent packet can benefit from the replies that
   have been received since the last packet was sent. As a result, MTU
   discovery converges much faster (2-3 times as fast, typically).

 - It reduces network spikiness - it's more network-friendly to send
   one packet from time to time as opposed to sending bursts.