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.

• Give Transport a concept of create-new vs overwrite
• Write lease files
• 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