Use PhabricatorCachedClassMapQuery in Conduit method lookups

Summary: Ref T11954. Depends on D16994. This implements the Conduit method cache described in that revision for a small global Conduit performance improvement.

Test Plan: Verified Conduit has the same behavior at lower cost. See D16994 for details.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11954

Differential Revision: https://secure.phabricator.com/D16995