Benji

Notable changes:

* Restores are now multi-threaded just like backups. This should speed things up quite a bit.

* Multi-threaded removal of blocks was implemented. This should speed up `benji cleanup` with B2. Due to a
simplification in the code `benji cleanup` for S3 based storages is probably slower than before. Try increasing
`simultaneousRemovals` in your configuration if you're affected by this. If this doesn't help, please
open an issue.

* The backup scripts got another major overhaul:

* It is now possible to hook into strategic points in the backup process with custom `bash` functions.
* Prometheus metrics were reimplemented with these hooks and are now specific to the `benji-k8s` Docker image.
* The foundations for freezing the filesystem before a Ceph snapshot were laid.
* The scripts in the `benji-k8s` Docker image now generate Kubernetes events about backup failure or
success. These events are attached to the affected PersistentVolumeClain and can for example be
viewed with `kubectl describe pvc`.
* `benji-backup-pvc` was renamed to `benji-pvc-backup`.
* Support for the try/catch construct based on the `bash-oo-framework` was removed. It had limitations
and was causing problems with certain IDEs.

* The default for simultaneous read and writes was increased from one to three to get better out-of-the-box
performance. The default for the simultaneous removals was set to five.

* Left over locks due to power outages are similar events can now be overridden. This applies to `benji rm` and
`benji cleanup`.

* A typo was fixed in the `metadata-backup` command. It is now correctly spelled `metadata-backup` and
not `netadata-backup` as before. User action might be required if you're using this command in your scripts.

* A bug was fixed where blocks where not properly cleaned up when multiple storages were in use.

* The documentation was updated, but we're still not up-to-date at all fronts.

Notable changes:

* Fix a big locking design problem.
* Convert `valid` boolean flag into a `status` field in versions table. This solves a problem where incomplete
backups where marked as valid by a `deep-scrub`. The new status field can be queried in filter expressions
like so: `status == "valid"`. Third party scripts might need adjustments.
* All binary keys and salts in the configuration file are now BASE64 encoded instead of using the `!!binary`
YAML extension. This is for better compatibility with 3rd party tools like Helm. User action needed.
* Implement proper handling of datetime columns in filter expressions. Things like `date < "1 day ago" `
are now possible.
* Make database migrations work properly again. Database migrations are now an explicit process and not automatic
anymore. Use the new `database-migrate` command.
* When restoring to an existing RBD image discard all existing data before the restore so as not to use more
space then needed.
* Increase standards conformance of Benji's NBD server (a little bit).
* Convert read cache to use a sharded design to increase scalability.
* Optimize database access pattern when backing up an image with a lot of sparse blocks.
* Optimize database commit handling.
* Implement workaround for SQLite database write access contention problem. Please use PostgreSQL if you're
planning on having multiple Benji processes running at the same time, SQLite wasn't designed for this.

A big thank you goes to olifre for his extensive testing efforts and his valuable feedback!