In the updates/delete code path we call createPredicatesOnBucketLevelField which can use the fixed buckets rewrites in these places:

1. getBucketLevelPredicateForRouting (won't apply for the optimization since this is on mongos).
2. getPushdownPredicates() --> getMatchExprsForWrites --> ParsedDelete and ParsedUpdateBase
3. getPushdownPredicates() --> pushdownPredicate --> 
   makeTimeseriesCreateIndexesCommand

We need to investigate if we can pass the fixedBuckets parameter to these places.