Added global ordinals terms aggregator impl that is optimized low cardinality fields.
Instead of resolving the global ordinal for each hit on the fly, resolve the global ordinals during post collect.
On fields with not so many unique values, that can reduce the number of global ordinals significantly.

Closes #5895
Closes #5854
Cut other aggregations over to use collectExistingBucket() if a bucket ord has been hit, that already exists.

Closes #5955