Aggs: Add cancellation checks to FilterByFilter aggregator

[ivancea]

By default, the FilterByFilterAggregator (Used by the "filter" and "filters" aggs) was using the DefaultBulkScorer (From Lucene), which has no cancellation mechanism.

This PR wraps it into a CancellableBulkScorer, which instead calls the inner scorer with ranges, and checks cancellation between them.

This should solve cases of long-running tasks using these aggregators not being cancelled, or greatly reduce the time they take after cancellation.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[elasticsearchmachine]

Hi @ivancea, I've created a changelog YAML for you.

[elasticsearchmachine]

Hi @ivancea, I've updated the changelog YAML for you.

[nik9000]

Could you use a pair of Semaphors like so:

private static final Semaphore scriptRunPermits = new Semaphore(0);
private static final Semaphore cancelRunPermits = new Semaphore(0);
...

In the test:
cancelRunPermits.acquire();
client().cancelTheRequest();
scriptRunPermits.release(Integer.MAX_VALUE);
...

In the script:
cancelRunPermits.release(1);
scriptRunPermits.acquire();

I'm sure there's a simpler way to do it. But this'd block the cancel task until the the script starts. Then block the script until the cancel has finished.

[ivancea]

Done! It looks far better, faster and consistent with a Repeat(1000)