Jobs should be marked as completed when all processes are completed, not just the last one

[ridiculousfish]

This issue tracks the following change: mark a job as completed only when all processes are finished.

An example of an affected command:

 sleep 10 | echo Hi

Today in fish, this finishes immediately, because echo does not read from its input pipe. With this change, this job will not finish for 10 seconds.

Advantages:

1. It's arguably more intuitive
2. It makes it possible to implement pipefail (add pipefail #763), because we will wait until we have the exit status of all commands in the pipeline
3. It eliminates a certain class of race conditions that occur if the last process is not in the process group of the session leader (pwd | pwd crashes #848)
4. It matches bash and zsh (though not tcsh)

[ridiculousfish]

Trying it and seeing how it works out. Fixed as 5d75ee7