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.

Closes T2773.

Do not unnecessarily format string (eg: when log statement is below
current log level).

Closes T2772.

During development of new features, enabling the DEBUG log-level every
time to hunt down a runtime exception is unpractical since the DEBUG
level is very verbose and does not include the exception traceback.
Using the logging.exception() method fixes all these issues.

The asizeof() introduced a very heavy overhead which made swhfs unusable
when relying on many asizeof calls (eg: storing history exploration in
the direntry cache with a history containing >1000 commits).

The heuristic to calculate the size of a FuseEntry is based on
experimental results from repositories with various history sizes (10K,
100k, ...)

Pre-fill the direntry cache with all the sub-entries once the history
has been retrieved.

Closes T2695.

The readdir() endpoint is now O(N) instead of O(N²) since we do not go
through all entries every time, but only those after the readdir()
offset.

Closes T2723.

Rework existing mock archive to account for a new graph backend.

Closes T2659.

Rationale: ease batch commit workloads.

Closes T2725.

rationale:
- it is consistent with SwhFS which we are now using in doc
- FUSE is an implementation detail, which has no place in user-facing names
- it is consistent with other FUSE-based FS (sshfs, RepoFS, etc.)

Closes T2708.

Closes T2716.

Closes T2711.

Closes T2709.

Closes T2702

without this click includes the full (long) docstring in the main --help

Closes T2669

Closes T2699.

h/t @haltode

also apply various other minor improvements to the configuration documentation:

- mention and show in the example the outer "swh > fuse" stanza
- fix sphinx markup for verbatim text (it's "``", not "`")
- add configuration file doc to the doc index

- Closes T2705: move --config-file to "swh fuse".
- Closes T2678: add default configuration location.