Support garbage collecting inactive branches

[ozkatz]

Extend lakeFS' garbage collection to handle abandoned branches. Ideally, let users set a policy on what qualifies as a GC-able branch:

1. it falls under a specific naming convention (i.e. starts with some defined prefix)
2. it had no writes/commits for a given duration of time
3. a certain amount of time has passed since its creation?
4. other criteria? (requires more discovery)

Inspired by this thread (Thanks Clinton!)