Instead of sending out disco keys via TSMP once, send them out in
intervals of 60+ seconds. The trigger is still callmemaaybe and the keys
will not be send if no direct connection needs to be established.
This fixes a case where a node can have stale keys but have communicated
with the other peer before, leading to an infinite DERP state.
Updates #12639
Signed-off-by: Claus Lensbøl <claus@tailscale.com>