Safekeepers should gossip remote_consistent_lsn via broker

[petuhovskiy]

I saw this happenning in tests:

1. pageserver updates remote_consistent_lsn to match last_record_lsn
2. connected safekeeper receives new LSN
3. safekeeper updates broker_is_active to false
4. it stops pushing updates to broker
5. other safekeepers have no chance to learn new remote_consistent_lsn from broker

The fix is to delay timeline deactivation for some time (30s), so that safekeepers would have a chance to broadcast remote_consistent_lsn update to peers. It's not a solution for 100% of cases, but should work good enough.

[jcsp]

(notes chatting with Arthur)

Impact: interferes with writing clean tests. Currently if a safekeeper has stale remote_consistent_lsn for long enough, it will remain active & the pageserver will eventually connect to it. When the pageserver connects it will eventually learn remote_consistent_lsn.

More generally: should we reconsider using remote_consistent_lsn in the safekeeper in our condition for broker_is_active?