Use a lease rather than `GC_LOCK`?

[sourcefrog]

https://github.com/sourcefrog/conserve/wiki/Leases-and-locks

In fact, for easier compatibility this could use the same filename, but just add an expiry time, and then repeatedly update it while running.

• [x] Give Transport a concept of create-new vs overwrite
• [x] Write lease files
• [x] Detect stolen/lapsed leases
• [ ] Test expiry, theft, etc
• [ ] Periodically renew leases
• [ ] Think about how to make this transition well from GC_LOCK
• [ ] Integrate leases into code that writes to the archive