R1:71d4fa41c93b

Support the Ferret "=" (exact match) operator in the actual query engine

Summary:
Ref PHI778. In D18492, I added support for parsing this operator, but did not actually implement it in the query engine.

Implementation is fairly straightforward. This supports querying for objects by exact title with `title:="exact title"`. This is probably a bad idea, but sometimes maybe useful anyway.

Test Plan: Queried for `title:="xxx"`, found only exact matches.

Reviewers: amckinley

Reviewed By: amckinley…