[job] implement ZX_JOB_TERMINATED signal

Implements ZX_JOB_TERMINATED, which is signaled when a job is killed,
after all its children have finished cleaning up.

This also changes job tracking behavior. Previously, if a job was killed
it would stay in the job trees until the last handle was closed. With
this patch, jobs now behave like threads or processes and remove
themselves from the task trees as soon as they (and all children) have
been killed. This provides a more consistent behavior among all tasks.

As a nice side-effect, now if a job fails to create (e.g. if the parent
is already dead) we never add it to the all-jobs tree; previously it
would be briefly added and then removed.

ZX-3919

Test: runtests
Test: run-zircon-x64 -C -b -k -z core-tests-x64.zbi
Change-Id: Ic367b7df9704e5012a1c73636d76784c527a4636