Use futures to improve clustered repository main page performance

Summary:
Ref T11954. In cluster configurations, we get repository information by making HTTP calls over Conduit.

These are slower than local calls, so clustering imposes a performance penalty. However, we can use futures and parallelize them so that clustering actually improves overall performance.

When not running in clustered mode, this just makes us run stuff inline.

Test Plan:

• Browsed Git, Mercurial and Subversion repositories.
• Locally, saw a 700ms wall time page drop to 200ms.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11954

Differential Revision: https://secure.phabricator.com/D17009