Delete query does not affect immature splits

[fmassot]

By nature, immature splits should not remain immature. With a given merge policy, they should become mature, one day. However, we don't know when they will become mature, and under certain assumptions, it may happen in a very long time. For example, if a merge policy has a huge split_num_docs_target... And in this case, we will never delete documents.

We can certainly assume that a user should know what he is doing but... delete queries are mainly here for GDPR reasons and it can be reassuring for a user to be able to easily check if his delete query has really deleted all documents everywhere or ensure that we delete documents even on immature splits.

At first, a clear documentation is needed, and according to feedback on this feature, we may fix (or not) this issue.