* fix tailscale status for peers using discovery
* as part of that, pull out disco address selection into reusable
and testable discoEndpoint.addrForSendLocked
* truncate ping/pong logged hex txids in half to eliminate noise
* move a bunch of random time constants into named constants
with docs
* track a history of per-endpoint pong replies for future use &
status display
* add "send" and " got" prefix to discovery message logging
immediately before the frame type so it's easier to read than
searching for the "<-" or "->" arrows earlier in the line; but keep
those as the more reasily machine readable part for later.
Updates #483
Ping messages now go out somewhat regularly, pong replies are sent,
and pong replies are now partially handled enough to upgrade off DERP
to LAN.
CallMeMaybe packets are sent & received over DERP, but aren't yet
handled. That's next (and regular maintenance timers), and then WAN
should work.
Updates #483
