Improve remarkup performance (Phabricator)

Summary: As we've moved more remarkup logic post-cache (e.g., to implement policy-aware rules), we're paying more of a price for it. Improve performance in Phabricator by sharing engine instances where possible.

Test Plan: With next diff, this drops 75% of the cost of markup handling in Conpherence.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D5510