Enhancement: speed up index loading

Loading the index for a large repository is now significantly faster. `restic mount`
now also loads the index once on startup and incrementally loads only
new index files afterwards. In addition, `restic mount` now loads snapshots
before printing that the repository is being served.

https://github.com/restic/restic/pull/5720
https://github.com/restic/restic/pull/5713