R1:b8f75f95116e

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…