Stop markup cache from hitting cache O(N^2) times

Summary:
The intent of getOrBuildEngine() is to save some boilerplate in cases where we're just using a standard engine, but it didn't get cached so we'd rebuilt it over and over again.

This was especially bad in Differential with a large number of inlines.

Test Plan: "Query" tab of services is no longer quite so ridiculous in Differential.

Reviewers: btrahan, dctrwatson

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D8352