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