After some time, #threads drop to 1 with --encoding BROTLI

[m-schuetz]

CPU usage during indexing initially at around 80%, drops down to 10% after about 5% of indexing is done.

• Could probably be an output bottleneck
• Is compress() (indexer.cpp) limited to one thread?
  • compress is writeAndUnload -> compress
• Or is the writer backlog not computed correctly for compressed data? (check waitUntilWriterBacklogBelow)