Recovery of .partial WAL segments

Dear hackers,

Generating a ".partial" WAL segment is pretty common nowadays (using pg_receivewal or during standby promotion).
However, we currently don't do anything with it unless the user manually removes that ".partial" extension.

The 028_pitr_timelines tests are highlighting that fact: with test data being being in 000000020000000000000003 and 000000010000000000000003.partial, a recovery following the latest timeline (2) will succeed but fail if we follow the current timeline (1).

By simply trying to fetch the ".partial" file in XLogFileRead, we can easily recover more data and also cover that (current timeline) recovery case.

So, this proposed patch makes XLogFileRead try to restore ".partial" WAL archives and adds a test to 028_pitr_timelines using current recovery_target_timeline.

As far as I've seen, the current pg_receivewal tests only seem to cover the archives generation but not actually trying to recover using it. I wasn't sure it was interesting to add such tests right now, so I didn't considered it for this patch.

Many thanks in advance for your feedback and thoughts about this,
Kind Regards,
--
Stefan FERCOT
Data Egret (https://dataegret.com)