Run all willLintPaths() before any lintPath()

Summary:
FB runs some linters on background and it's a magnificent hack using `ParallelLinter` (two instances), `BackgroundLinter` and `FutureLinter`.
I want to simplify this by resolving the futures in engine instead of in some virtual linter.
It also seems like a better place to do it.
It should also fix caching problems I have with them (because the virtual linters don't know about the cache at all).

Test Plan: None yet.

Reviewers: epriestley, nh

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4380