[flow] add chunk size to toml settings

Also, implement pure `mapConcurrently` in extract ngrams, as it's
closely related to chunk size.
4 jobs for dev-threaded-flow-poc in 114 minutes and 37 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Cabal
passed #19459
cabal

01:17:50

 
  Stack
passed #19460
stack

00:03:59

 
  Bench
manual #19461
allowed to fail manual
bench
 
  Test
passed #19462
test

00:32:46