Fixes T2838.

Related to T2771.

Add a new 'cache/' directory along 'archive/' and 'origins/' in order to
navigate previously cached artifacts and for a future use-case about
cache invalidation on specific objects.

Close T2863.

Fix T2842.

Fix T2851.

See details and discussion in T2771.

- ls archive/ is now always an empty result
- Merge the meta/ directory content into archive/

Fix T2840.

Fix T2839.

The NamedTemporaryFile was being closed twice, once at the end of
fuse_process() and the other at the end of fuse_mntdir().

Closes T2820.

- Explicit data types in tables
- Add comments
- String formatting

Closes T2829. The ".status" file presence is currently not a reliable
system to stop the delay, so use a fixed one instead to prevent random
CI fails. The .status issue is now tracked in T2830.

Closes T2803.

Before, ls history/by-date/ took a few seconds and it was getting slower
the more metadata were stored in the sqlite cache. This was because it
would query the metadata cache for all SWHIDs in history.

Now, a new column has been created to store a relation (rev swhid, date)
meaning we only need to do one query to get all history with a specific
date prefix.

Closes T2802.

Closes T2784.

Closes T2676

Closes T2785

most notably drop "virtual" from filesystem everywhere for consistency

- Remove manual direntry cache manipulation
- Remove unnecessary by-hash/ sharding depth attribute
- Time optimization for by-date/ partial results
- More extensive unit testing for by-date/

Closes T2795.