Cleaned up initialization of atomic fields in ProcessManager.
Cleaned up initialization of atomic fields in ProcessManager.

`std::atomic<T>` has a trivial default constructor, which means
`std::atomic` values are left uninitialized by default. `ProcessManager`
uses two atomic fields that are default-constructed; although we `store`
to them before reading from them, it would be cleaner and safer to
initialize them to a well-defined value in the first place.

Review: https://reviews.apache.org/r/51984/

chhsia0/mesos,dforsyth/mesos,zmalik/mesos,neilconway/mesos,abudnik/mesos,asamerh4/mesos,andschwa/mesos,neilconway/mesos,craimbert/mesos,kaysoky/mesos,chhsia0/mesos,jpeach/mesos,jpeach/mesos,craimbert/mesos,Gilbert88/mesos,zmalik/mesos,yuquanshan/customizedMesos,asamerh4/mesos,dforsyth/mesos,Aman-Jain-14/customizedMesos,dforsyth/mesos,yuquanshan/customizedMesos,Gilbert88/mesos,chhsia0/mesos,abudnik/mesos,chhsia0/mesos,neilconway/mesos,andschwa/mesos,gsantovena/mesos,dforsyth/mesos,abudnik/mesos,abudnik/mesos,yuquanshan/customizedMesos,neilconway/mesos,verizonlabs/mesos,Aman-Jain-14/customizedMesos,reneploetz/mesos,kaysoky/mesos,yuquanshan/customizedMesos,yuquanshan/customizedMesos,asamerh4/mesos,gsantovena/mesos,Aman-Jain-14/customizedMesos,Aman-Jain-14/customizedMesos,kaysoky/mesos,gsantovena/mesos,craimbert/mesos,jpeach/mesos,verizonlabs/mesos,gsantovena/mesos,gsantovena/mesos,craimbert/mesos,andschwa/mesos,reneploetz/mesos,yuquanshan/customizedMesos,kaysoky/mesos,kaysoky/mesos,asamerh4/mesos,jpeach/mesos,verizonlabs/mesos,reneploetz/mesos,reneploetz/mesos,Aman-Jain-14/customizedMesos,kaysoky/mesos,craimbert/mesos,abudnik/mesos,asamerh4/mesos,kaysoky/mesos,chhsia0/mesos,craimbert/mesos,andschwa/mesos,andschwa/mesos,reneploetz/mesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,neilconway/mesos,reneploetz/mesos,verizonlabs/mesos,reneploetz/mesos,gsantovena/mesos,andschwa/mesos,craimbert/mesos,yuquanshan/customizedMesos,Gilbert88/mesos,neilconway/mesos,andschwa/mesos,zmalik/mesos,dforsyth/mesos,Gilbert88/mesos,jpeach/mesos,dforsyth/mesos,zmalik/mesos,dforsyth/mesos,chhsia0/mesos,zmalik/mesos,zmalik/mesos,jpeach/mesos,verizonlabs/mesos,gsantovena/mesos,asamerh4/mesos,abudnik/mesos,jpeach/mesos,asamerh4/mesos,Gilbert88/mesos,zmalik/mesos,verizonlabs/mesos,Gilbert88/mesos,neilconway/mesos,abudnik/mesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,chhsia0/mesos