Garbage collection of old relationship history.

Licet keeps deleted and expired relationships, and the snapshots of past writes, for gc_window seconds, so that tokens from that window can still be read exactly. Every instance runs a collector that removes older history every gc_interval milliseconds, in batches of gc_batch rows.

On several nodes, each pass takes a Postgres advisory lock, so only one node collects at a time. Set gc_interval: false to disable automatic passes, for example in tests or to trigger them from your own scheduler with collect/1.