Implement garbage collection in the oplog

[garethgeorge]

Collect and delete old entries in the operations log. An entry can be deleted if it is pushed out of the last 100 operations for any repo or plan.

[garethgeorge]

Something better may be needed in the future. For now I've compromised by explicitly deleting snapshots that are marked as removed by a prune operation. Implicitly deleted (e.g. no longer found after running restic snapshots or such) operations will still be retained to avoid data loss e.g. to avoid dataloss if snapshot output can't be parsed correctly.