split SIGCHLD from all other signal handlers in supervisor (v2)

[tgross]

For #490, in v2

This PR splits out the goroutine that handles the SIGCHLD signal from all other signal handlers. This lets us loop on that handler without worrying about spawning additional goroutines for each signal received and should prevent resource growth in the case where many SIGCHLD signals land during the handling of one of them.

cc @cheapRoc @roengram

[tgross]

For some reason the tasks test is now failing. I would say it's flaky in v2 (which is certainly a possibility) but it's failing in a consistent way. Lemme look into that:

2017/08/22 15:30:50 TEST: test_tasks
Creating testtasks_consul_1
testtasks_consul_1 is up-to-date
Creating testtasks_app_1
Expected task1 to have 7 or 8 executions: got 6
No stopped containers
2017/08/22 15:30:57 FAIL: test_tasks