R1:1de130c9f5b2

Allow the Ferret engine to remove "common" ngrams from the index

Summary:
Ref T13000. This adds support for tracking "common" ngrams, which occur in too many documents to be useful as part of the ngram index.

If an ngram is listed in the "common" table, it won't be written when indexing documents, or queried for when searching for them.

In this change, nothing actually writes to the "common" table. I'll start writing to the table in a followup change.

Specifically, I plan to do this:

- A new GC…