[ngrams] highlight caching

Also, limit to context ngrams only, for searching highlight patterns.
2 jobs for 551-dev-annotation-fix in 0 seconds (queued for 60 minutes and 12 seconds)
Status Job ID Name Coverage
  Test
failed #7186
deps

failed #7187
test

 
Name Stage Failure
failed
test Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job trace
failed
deps Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job trace