R1:ec6d69e74d92

Give Drydock resources a proper expiry mechanism

Summary:
Fixes T6569. This implements an expiry mechanism for Drydock resources which parallels the mechanism for leases.

A few things are missing that we'll probably need in the future:

- An "EXPIRES" command to update the expiration time. This would let resources be permanent while leased, then expire after, say, 24 hours without any leases.
- A callback like `shouldActuallyExpireRightNow()` for resources and leases that lets them decide not to…