This commit introduces the `OnLoopDetectedStrategy` type and does a
slight refactoring so that we can "plug in" algorithms to break loops
during our forest search.

It also introduces a test, which at the moment is failing, that we
can actually break a loop.

[The previous revert was a wrong track] Revert "Merge branch 'revert-97441f3d' into 'dev'"

See merge request !450

This reverts merge request !449

Revert "Merge branch 'adinapoli/issue-504' into 'dev'"

See merge request !449

This reverts merge request !439

Dev allow for api error in flow

See merge request !440

Dev worker fixes

See merge request !447

Dev add option to notify users

See merge request !438

Keep only the roots in searchTableNgrams

See merge request !439

[query] a prefixed query like ~prefix should transform to 'prefix', not '"prefix'

See merge request !443

This prevents multiple workers getting the same job at the moment it
times out.

In particular:

* In ngrams search tests where we were comparing the total number of
  results returned, we need to account only for the roots, because the
  total number we would be returning from the pagination would be
  higher, as it would include children, in some cases;
* In the `AcyclipTableMap` generator, the roots we generate need to be
  so (i.e. `_ne_root` and `_ne_parent` must be `Nothing`).

After we rewrote `destroyForest`, it means that in normal conditions we
are going to output roots followed by their nodes (in in-order
ordering), so the test outputs need adjustments.

Destroying a forest should flatten any recursive children, or they won't
show up in the final ngrams table.

add missing module

See merge request !444

Add the ability to emit logging messages from a `DbTx` transaction

Closes #499

See merge request !436

Prevent importing ngrams which will lead to loops

See merge request !435

Added tests.

[CLI] db fix command, to fix hyperdata #630

See merge request !442

[search] small refactoring of the search API

See merge request !441

Dry run mode enabled by default

[API] version with git hash

See merge request !422

Continue with the flow and import terms.

This way, a failed API call doesn't prevent from the job to finish
gracefully.

TODO: Print a nice, formatted error

[cli] add support to spawn multiple workers of the same type

See merge request !437

Also, added Data.List.nubBy to limit to only 1 dispatched notification
per ws connection. This should fix a bug whereby a single user got
multiple notifications about the same thing.