Fix ephemeral crash/hang on termination due to sigchild handler
Fix ephemeral crash/hang on termination due to sigchild handler

The sigchld handler for reaping child process relies on a global
std::set which does process id bookkeeping. The global variable
could be destroyed before the the signal handler is called, causing
undefined behavior on termination.

turi-code/SFrame,TobyRoseman/SFrame,hoytak/SFrame,ylow/SFrame,turi-code/SFrame,haijieg/SFrame,turi-code/SFrame,dato-code/SFrame,ylow/SFrame,dato-code/SFrame,TobyRoseman/SFrame,turi-code/SFrame,turi-code/SFrame,haijieg/SFrame,haijieg/SFrame,TobyRoseman/SFrame,ylow/SFrame,dato-code/SFrame,TobyRoseman/SFrame,TobyRoseman/SFrame,hoytak/SFrame,hoytak/SFrame,dato-code/SFrame,dato-code/SFrame,hoytak/SFrame,ylow/SFrame,turi-code/SFrame,ylow/SFrame,haijieg/SFrame,haijieg/SFrame,dato-code/SFrame,haijieg/SFrame,ylow/SFrame,hoytak/SFrame,hoytak/SFrame,haijieg/SFrame,dato-code/SFrame,haijieg/SFrame,TobyRoseman/SFrame,dato-code/SFrame,turi-code/SFrame,hoytak/SFrame,TobyRoseman/SFrame,ylow/SFrame,hoytak/SFrame,turi-code/SFrame,TobyRoseman/SFrame,ylow/SFrame