We did spawn exactly one task for each of copy&reloc and publishing.
Those tasks did block until work is available. This can block background
threads which could otherwise execute other component's work.
Switching to the Job API allows us to easily avoid that blocking, and
just respawning a task when more work is available.
Is always avoid code duplication for participating in the work in the
main thread. Instead we just {Join()} the existing job, which makes the
current thread participate in work.

For now, both Jobs set a maximum concurrency of one, so the main thread
will only do work if no background thread is currently running. This can
be lifted in a follow-up CL to see the performance impact of both
changes independently.

R=thibaudm@chromium.org

Bug: v8:11164
Change-Id: I032153eb933648a750b113f5d766feb85b87070a
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_isolates_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2643393Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72288}