Restructure pg_upgrade output directories for better idempotence

38bfae3 has moved the contents written to files by pg_upgrade under a
new directory called pg_upgrade_output.d/ located in the new cluster's
data folder, and it used a simple structure made of two subdirectories
leading to a fixed structure: log/ and dump/.  This design has made
weaker pg_upgrade on repeated calls, as we could get failures when
creating one or more of those directories, while potentially losing the
logs of a previous run (logs are retained automatically on failure, and
cleaned up on success unless --retain is specified).  So a user would
need to clean up pg_upgrade_output.d/ as an extra step for any repeated
calls of pg_upgrade.  The most common scenario here is --check followed
by the actual upgrade, but one could see a failure when specifying an
incorrect input argument value.  Removing entirely the logs would have
the disadvantage of removing all the past information, even if --retain
was specified at some past step.

This result is annoying for a lot of users and automated upgrade flows.
So, rather than requiring a manual removal of pg_upgrade_output.d/, this
redesigns the set of output directories in a more dynamic way, based on
a suggestion from Tom Lane and Daniel Gustafsson.  pg_upgrade_output.d/
is still the base path, but a second directory level is added, mostly
named after an ISO-8601-formatted timestamp (in short human-readable,
with milliseconds appended to the name to avoid any conflicts).  The
logs and dumps are saved within the same subdirectories as previously,
as of log/ and dump/, but these are located inside the subdirectory
named after the timestamp.

The logs of a given run are removed only after a successful run if
--retain is not used, and pg_upgrade_output.d/ is kept if there are any
logs from a previous run.  Note that previously, pg_upgrade would have
kept the logs even after a successful --check but that was inconsistent
compared to the case without --check when using --retain.  The code in
charge of the removal of the output directories is now refactored into a
single routine.

Two TAP tests are added with some --check commands (one failure case and
one success case), to look after the issue fixed here.  Note that the
tests had to be tweaked a bit to fit with the new directory structure so
as it can find any logs generated on failure.  This is still going to
require a change in the buildfarm client for the case where pg_upgrade
is tested without the TAP test, though, but I'll tackle that with a
separate patch where needed.

Reported-by: Tushar Ahuja
Author: Michael Paquier
Reviewed-by: Daniel Gustafsson, Justin Pryzby
Discussion: https://postgr.es/m/77e6ecaa-2785-97aa-f229-4b6e047cbd2b@enterprisedb.com

doc/src/sgml/ref/pgupgrade.sgml

@@ -768,7 +768,10 @@ psql --username=postgres --file=script.sql postgres
   <para>
    <application>pg_upgrade</application> creates various working files, such
    as schema dumps, stored within <literal>pg_upgrade_output.d</literal> in
-   the directory of the new cluster.
+   the directory of the new cluster. Each run creates a new subdirectory named
+   with a timestamp formatted as per ISO 8601
+   (<literal>%Y%m%dT%H%M%S</literal>), where all the generated files are
+   stored.
   </para>
 
   <para>