Inefficient index updates cause high cost merges and increase overall latency

Currently every change to the index is done Lucene-wise combining two operations:

• Delete by query, using a boolean query on the id plus the entity class
• Add

Under high load, specially during merges those numerous deletes provoke very long delays causing high latency.

We should instead use a simple Lucene Update to add/change documents, since internally it translates to a Delete by term plus an Add operation, and delete by terms are extremely efficient in Lucene.

Some local tests showed average latency of updating the index using this strategy to drop 4 times, both for the SYNC and ASYNC backends

With relation to sharing the index between entities, which was the original motivation of the Delete by query plus add strategy, we have two scenarios:

• Same cache with multiple entity types: that's a non-issue, since obviously there's no id collision in this case
• Different caches with the same index: this scenario happens when different caches shares the same index, for ex:

@Indexed(indexName=common)
public class Country { ... }

@Indexed(indexName=common)
public class Currency { ... }

cm.getCache("currencies").put(1, new Currency(...))
cm.getCache("countries").put(1, new Country(...))

This would require a delete by query in order to persist both a Country and a Currency with id=1.
It would also require setting "default.exclusive_index_use", "false", with the associated cost of having to reopen the IndexWriter on every operation.

Given the performance gain of doing a simple Update is considerable, we should make the corner case supported by extra configuration or alternatively, generate a unique @ProvidedId, including the entity class or the cache name that work for all cases described above.