Renew config message TTLs at most once an hour per swarm

[mpretty-cyro]

Every poll asked the swarm to extend the TTL of every active config message. That is a write on every storage node holding those messages, every few seconds, from every client, and service nodes see real disk I/O from it. The extension goes to weeks from now, so doing it once an hour loses nothing.

The change

A poll now skips the config TTL extension for a swarm whose last extension succeeded within the last hour.

• Per swarm. The user swarm and each group swarm have their own cooldown, so one group's renewal never suppresses another's or the user's own.
• Success only. A failed extension leaves the cooldown untouched, so the next poll retries it. A failure that started the cooldown would leave the configs un-renewed while looking handled, and repeated failures could let them age out of the swarm.
• In memory, one hour, constant. No persistence or migration. A restart re-arms it, so the first poll after launch always extends.
• A throttled poll sends no expire request at all, rather than sending one and ignoring the answer.

Disappearing-message expiry uses a separate path and is unchanged.

iOS specifics

• Network.StorageServer.ConfigTtlExtensionThrottle (actor, .configTtlExtensionThrottle singleton) holds the per-swarm timestamps.
• StorageServer.poll decides whether to include the expire sub-request, and records success only when that sub-response parsed and returned 2xx.
• Timing uses dependencies.dateNow. A clock moved backwards counts as due, so a clock change can never hold the cooldown open.

Verification

Seven new specs in SwarmPollerSpec ("a swarm poll refreshing config TTLs") drive the real StorageServer.poll against MockNetwork. They count the batch bodies that actually carry an expire sub-request, so they check what is sent, not what the caller passed. Covered: two polls inside the window send one; a poll after the window sends another; a 500, an unparseable body, or a failed batch don't start the cooldown; a backwards clock; per-swarm independence.

SwarmPollerSpec 11/11 via the Session scheme (-parallel-testing-enabled NO). With the 2xx check removed, the "extension fails" spec goes red.

Same change in the other clients

• Renew config message TTLs at most once an hour per swarm session-android#2225
• fix: renew config message TTLs at most once an hour per swarm session-desktop#2017