Implement "Wait for Previous Builds" build step

Summary: This adds a build step which will block a build from continuing if there are previous builds of the build plan still running.

Test Plan: Configured a build plan with a wait of 60 seconds and a "wait for previous builds", then started a build. While that was still building, reconfigured the plan to have a wait time of 3 seconds, started it, and saw it move into the "Waiting" status. When the 60 second build finished, both builds passed.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7745