Stop reaping children from SIGCHLD signal handler

Prior to this fix, a child process may be reaped in one of two ways:
1. By a call to waitpid() within job_continue
2. By a call to waitpid() within the SIGCHLD signal handler

Only the second call was with the WNOHANG option. Thus if the signal
handler fired first, and then the waitpid call fired, we could get a
deadlock because we'd end up waiting on a long-running process. I have
not been able to reproduce this on fish 1.x, though it seems like it
ought to reproduce there too.

This fix migrates the waitpid() call out of the signal handler; the
second class of calls moves to job_reap. This eliminates the possibility
of a race, because we check for job completion before calling waitpid,
and there is no longer the possibility of the job being marked as
complete asynchronously. It also results in a massive conceptual
simplification, since the signal handler is now very simple and easy to
reason about (no more walking jobs lists, etc).

This partially fixes a bug reported in #1273