pgsql: Improve handling and coverage of --no-ensure-shutdown in pg_rewi

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

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/caa078353ecd1f3b3681c0d4fa95ad4bb8c2308a

Modified Files
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doc/src/sgml/ref/pg_rewind.sgml | 10 ++++++----
src/bin/pg_rewind/pg_rewind.c | 19 ++++++++++++-------
src/bin/pg_rewind/t/005_same_timeline.pl | 4 ++++
src/bin/pg_rewind/t/RewindTest.pm | 20 ++++++++++++--------
4 files changed, 34 insertions(+), 19 deletions(-)