Bypass message queue when it risks a collision with SIGCHLD.

Summary:
This Saturday, we've got SIGCHLD caught by rabbitmq.so at
00:00, 1:00, 1:30, 5:55, 6:55 11:50 and 12:20.

So every 5 minutes, when it's time for SIGCHLD, we avoid to
call mq get.

Fixes T627.

Test Plan: Wait 24h, check if we've still some SIGCHLD.

Reviewers: dereckson

Maniphest Tasks: T627

Differential Revision: http://devcentral.nasqueron.org/D111