Migrate ProjectGenerationStatPublisher to Elasticsearch bulk calls

[bclozel]

Instead of sending one request per project generation, we should leverage the Elasticsearch high level REST client and send bulk requests with many stats at once.

[bclozel]

Would be better if we rely on an elasticsearch version containing this fix elastic/elasticsearch#41338.
Before that fix, the BulkProcessor implementation retries bulk requests when the server fails processing them and responds with an error/acknowledges for each. But if the server is not available, the process fails hard and the retry is not considered.

Given that network errors are quite possible, I'd rather use Eslasticsearch 7.2.0+ (at least on the client side). Given our API exposure, I don't think we'd need to make that a hard requirement for developers using the library, just a preference of ours for production use.

[snicoll]

@bclozel do you have a branch somewhere with this?