Crash at system time change back

[lifsong]

There's a reproduce path to make supervisord crash:

1. set system time 1 year before real time.
2. killall one_program_under_supervisoring.
3. you'll see supervisord crashed.

Checked the code and see that our process state calculation depends a lot on system time. System time change may cause assertion.

[lonetwin]

Just adding a me-too here. We ran into this problem when running supervisord within a vmware instance which had an option set which would sync the guest's time with the host's. This would cause problems when the host was under load or the time ^jumped back^ due to ntp. If we then would kill one of the processes in the guest running under the supervisor instance on the guest, supervisor would die and orphan all the rest of the processes. I can confirm that this patch appears to stop supervisor from crashing.