Improve handling and coverage of --no-ensure-shutdown in pg_rewind

Improve handling and coverage of --no-ensure-shutdown in pg_rewind

This includes a couple of changes around the new behavior of pg_rewind
which enforces recovery to happen once on a cluster not shut down
cleanly:
- Some comments and documentation improvements.
- Shutdown the cluster to rewind with immediate mode in all the tests,
this allows to check after the forced recovery behavior which is wanted
as new default.
- Use -F for the forced recovery step, so as postgres does not use
fsync.  This was useless as a final sync is done once the tool is done.

Author: Michael Paquier
Reviewed-by: Alexey Kondratov
Discussion: https://postgr.es/m/20191004083721.GA1829@paquier.xyz