timersInit sets up the timers.
It need only be done once per peer.
timersStart does the work to prepare the timers
for a newly running peer. It needs to be done
every time a peer starts.
Separate the two and call them in the appropriate places.
This prevents data races on the peer's timers fields
when starting and stopping peers.
Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
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