Cancelling an indexing task does not always stop indexing quickly

[loiclec]

In Meilisearch v1.1, cancelling an indexing task is not guaranteed to stop the task quickly.
There are a few indexing phases which do not listen for the cancellation signal (for example, within the data extractors) and therefore a user may have to wait a very long time for indexing to stop. See this bug report.

We should try to implement cancellation during every indexing phase so that cancelling an indexing task immediately stops it.

[dureuill]

Ping from triage

• Severity: very low
• Size: medium to discover and fix some low hanging fruits. Very difficult to ensure that we won't introduce other bottlenecks in the future. Maintenance work.
• impact: performance degradation, increased latency to work on new tasks

[curquiza]

FYI: also this issue has been opened related to the topics #4358