(2/7) Let an instance mark a suspended slot expired

What does this MR do?

Instance-level groundwork, no public API change:

• Instance.ExpireSlot(slot, key, now, grace) marks a suspended slot Expired once now is more than grace past its expiry, and persists it. Slots without an expiry never expire.
• ResumeSlot refuses an expired slot.
• Capacity() still counts an expired slot as suspended, and reports how many are expired in a new Expired field.

Why was this MR needed?

Part 2 of 7. The taskscaler-level ExpireSuspended in the next MR is built on this.

What's the best way to test this MR?

go test -race ./internal/.... TestExpireSlot, TestCapacityCountsExpiredAsSuspended, and the "slot expired" case in TestResumeSlotErrors.

What are the relevant issue numbers?

Part of the stack started in !117 (merged).

MR stack

Merge in order:

1. (1/7) Give acquired slots an optional expiry (!117 - merged) • Ashvin Sharma
2. (2/7) Let an instance mark a suspended slot exp... (!119 - merged) • Ashvin Sharma (this MR)
3. (3/7) Expire suspended slots past their expiry (!120 - merged) • Ashvin Sharma
4. (4/7) Expose connect info for an expired slot's... (!121 - merged) • Ashvin Sharma
5. (5/7) Free expired slots once their workload is... (!122 - merged) • Ashvin Sharma
6. (6/7) Remove stopped instances whose slots have... (!123 - merged) • Ashvin Sharma
7. (7/7) Refuse to resume a suspended slot past it... (!118 - merged) • Ashvin Sharma