Ensure that a standby is able to follow a primary on a newer timeline.

 Ensure that a standby is able to follow a primary on a newer timeline.
 
 Commit 709d003fbd refactored WAL-reading code, but accidentally caused
 WalSndSegmentOpen() to fail to follow a timeline switch while reading from
 a historic timeline. This issue caused a standby to fail to follow a primary
 on a newer timeline when WAL archiving is enabled.
 
 If there is a timeline switch within the segment, WalSndSegmentOpen() should
 read from the WAL segment belonging to the new timeline. But previously
 since it failed to follow a timeline switch, it tried to read the WAL segment
 with old timeline. When WAL archiving is enabled, that WAL segment with
 old timeline doesn't exist because it's renamed to .partial. This leads
 a primary to have tried to read non-existent WAL segment, and which caused
 replication to faill with the error "ERROR:  requested WAL segment ... has
  already been removed".
 
 This commit fixes WalSndSegmentOpen() so that it's able to follow a timeline
 switch, to ensure that a standby is able to follow a primary on a newer
 timeline even when WAL archiving is enabled.
 
 This commit also adds the regression test to check whether a standby is
 able to follow a primary on a newer timeline when WAL archiving is enabled.
 
 Back-patch to v13 where the bug was introduced.
 
 Reported-by: Kyotaro Horiguchi
 Author: Kyotaro Horiguchi, tweaked by Fujii Masao
 Reviewed-by: Alvaro Herrera, Fujii Masao
 Discussion: https://postgr.es/m/20201209.174314.282492377848029776.horikyota.ntt@gmail.com