Defer suspend until launch completes

Insure that a job is completely launched before trying to suspend it.
Previous logic would start suspend logic early in the life of the
slurmstepd process, after it's listening socket was open but before
the tasks were launched. This defers the suspend logic until after
all prologs and setup completes and the tasks are launched. This is
important in the case of gang scheduling, in which newly launched
jobs can be immediately suspended.
bug 2494