* SOLR-14588: Implement Circuit Breakers
This commit consists of two parts: initial circuit breakers infrastructure and real JVM memory based
circuit breaker which monitors incoming search requests and rejects them with SERVICE_TOO_BUSY error
if the defined threshold is breached, thus giving headroom to existing indexing and search requests
to complete.
Add support for BlockMax WAND via a minExactHits parameter. Hits will be counted accurately at least until this value, and above that, the count will be an approximation. In distributed search requests, the count will be per shard, so potentially the count will be accurately counted until numShards * minExactHits. The response will include the value numFoundExact which can be true (The value in numFound is exact) or false (the value in numFound is an approximation).
Previous situation:
* The snowball base classes (Among, SnowballProgram, etc) had accumulated local performance-related changes. There was a task that would also "patch" generated classes (e.g. GermanStemmer) after-the-fact.
* Snowball classes had many "non-changes" from the original such as removal of tabs addition of javadocs, license headers, etc.
* Snowball test data (inputs and expected stems) was incorporated into lucene testing, but this was maintained manually. Also files had become large, making the test too slow (Nightly).
* Snowball stopwords lists from their website were manually maintained. In some cases encoding fixes were manually applied.
* Some generated stemmers (such as Estonian and Armenian) exist in lucene, but have no corresponding `.sbl` file in snowball sources at all.
Besides this mess, snowball project is "moving along" and acquiring new languages, adding non-BSD-licensed test data, huge test data, and other complexity. So it is time to automate the integration better.
New situation:
* Lucene has a `gradle snowball` regeneration task. It works on Linux or Mac only. It checks out their repos, applies the `snowball.patch` in our repository, compiles snowball stemmers, regenerates all java code, applies any adjustments so that our build is happy.
* Tests data is automatically regenerated from the commit hash of the snowball test data repository. Not all languages are tested from their data: only where the license is simple BSD. Test data is also (deterministically) sampled, so that we don't have huge files. We just want to make sure our integration works.
* Randomized tests are still set to test every language with generated fake words. The regeneration task ensures all languages get tested (it writes a simple text file list of them).
* Stopword files are automatically regenerated from the commit hash of the snowball website repository.
* The regeneration procedure is idempotent. This way when stuff does change, you know exactly what happened. For example if test data changes to a different license, you may see a git deletion. Or if a new language/stopwords/test data gets added, you will see git additions.
