Improve Conduit performance for custom fields

Summary:
Ref T11404. Depends on D16350.

Currently, custom fields can issue "N+1" queries in some cases, so querying 100 revisions issues 100 extra queries.

This affects all *.search endpoints for objects with custom fields, and some older endpoints (notably differential.query).

This change bulk loads "normal" custom fields, which gets rid of some of these queries. Instead of loading fields for each object, we build a big list of all fields and load them all at once.

The next change will tackle the remaining inefficient edge queries.

Test Plan:

• Configured a custom field with normal database storage in Differential.
• Ran differential.query, looking at custom fields in results for correctness.
• Ran differential.revision.search, looking at custom fields in results for correctness.
• In both cases, observed queries drop from 3N to 2N (all the "normal" custom field stuff got bulk loaded).

Reviewers: yelirekim, chad

Reviewed By: chad

Maniphest Tasks: T11404

Differential Revision: https://secure.phabricator.com/D16351