Currently we send one request per index, but that is not necessary: Elasticsearch allows targeting multiple indexes in one request.
This change would require moving these operations to IndexScope and creating multi-index works.
I started some work on one of my branches as part of HSEARCH-3049, but it turned out to not be necessary after all. See https://github.com/yrodiere/hibernate-search/tree/HSEARCH-3596 . The implementation is unfinished: it replaces PojoScopeWorkExecutor with a multi-index IndexScopeWorkExecutor, but instead it should have PojoScopeWorkExecutor delegate to multiple IndexScopeWorkExecutor, one per backend, so as to support multiple backends.