scion takes up to 30s to recover after a failed link recovers

[kormat]

There is an series of timers that works badly together in the case of a link recovery:

• The upstream BS has issued a revocation, which can last for up to 10s after the link comes back, during which time no beacons can be propagated.
• The upstream BS has a propagation timer of 5s, so it can take up to 5s after the revocation is expired before it propagates a beacon.
  That's 15s. The same applies to the downstream AS (except there it's the registration timer instead of the propagation timer).

[kormat]

Things we can do to reduce this window:

• Have both BSes preemptively clear revocations from BRs when a link comes back up, instead of waiting for them to expire.
• Have an BS propagate immediately when a downstream link comes back up, regardless of it's propagation timer.
• Have a BS register new beacons ~immediately when an upstream link comes back up and beacons are received.

[oncilla]

fixed by #2672