Enhancement: Add local metadata cache

We've added a local cache for metadata so that restic doesn't need to load
all metadata (snapshots, indexes, ...) from the repo each time it starts. By
default the cache is active, but there's a new global option `--no-cache`
that can be used to disable the cache. By default, the cache a standard
cache folder for the OS, which can be overridden with `--cache-dir`.  The
cache will automatically populate, indexes and snapshots are saved as they
are loaded. Cache directories for repos that haven't been used recently can
automatically be removed by restic with the `--cleanup-cache` option.

A related change was to by default create pack files in the repo that contain
either data or metadata, not both mixed together. This allows easy caching of
only the metadata files. The next run of `restic prune` will untangle mixed
files automatically.

https://github.com/restic/restic/pull/1040
https://github.com/restic/restic/issues/29
https://github.com/restic/restic/issues/738
https://github.com/restic/restic/issues/282
https://github.com/restic/restic/pull/1287
https://github.com/restic/restic/pull/1436
https://github.com/restic/restic/pull/1265
