Fixes #5395
Alternative to #5957

Make task restarting asynchronous when handling check-based restarts.
This matches the pre-0.9 behavior where TaskRunner.Restart was an
asynchronous signal. The check-based restarting code was not designed
to handle blocking in TaskRunner.Restart. 0.9 made it reentrant and
could easily overwhelm the buffered update chan and deadlock.

Many thanks to @byronwolfman for his excellent debugging, PR, and
reproducer!

I created this alternative as changing the functionality of
TaskRunner.Restart has a much larger impact. This approach reverts to
old known-good behavior and minimizes the number of places changes are
made.

Co-Authored-By: Mahmood Ali <mahmood@hashicorp.com>