IndexWriter with no threads

[fulmicoton]

The IndexWriter currently starts several threads, each one holding SegmentIndexWriter, each consuming the same mpmc channel.

This contraption exists even in single thread, and is somewhat justified by the possibility that a segment "spills".
If the segment writer exceeds its memory budget, it is serialized to disk and a new segment writer is created.
Without threading, the add_document call that will trigger this operation will block for a long time.