Hardcoded NUM_MERGE_THREADS

[Barre]

Hello,

I am trying to index a few billions documents.
In my usecase, the bottleneck seems to be the number of merging threads which seems to be hardcoded here: https://github.com/quickwit-oss/tantivy/blob/main/src/indexer/segment_updater.rs#L28

Would you be open to a PR that makes this configurable?

[PSeitz]

Yes, that could be configurable

[fulmicoton]

agreed.