Cache the computation of the custom block component of the markup key

Summary:
Caught this taking way too long on a production profile:

https://secure.phabricator.com/xhprof/profile/PHID-FILE-vfzq3sregh5xvpf5nc2t/?symbol=PhabricatorMarkupEngine::getMarkupFieldKey

Cache it; it's always identical.

Test Plan: Loaded Conpherence locally.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7583