pg_rewind: warn when checkpoint hasn't happened after promotion

A few weeks back I sent a bug report [1] directly to the -bugs mailing
list, and I haven't seen any activity on it (maybe this is because I
emailed directly instead of using the form?), but I got some time to
take a look and concluded that a first-level fix is pretty simple.

A quick background refresher: after promoting a standby rewinding the
former primary requires that a checkpoint have been completed on the
new primary after promotion. This is correctly documented. However
pg_rewind incorrectly reports to the user that a rewind isn't
necessary because the source and target are on the same timeline.

Specifically, this happens when the control file on the newly promoted
server looks like:

    Latest checkpoint's TimeLineID:       4
    Latest checkpoint's PrevTimeLineID:   4
    ...
    Min recovery ending loc's timeline:   5

Attached is a patch that detects this condition and reports it as an
error to the user.

In the spirit of the new-ish "ensure shutdown" functionality I could
imagine extending this to automatically issue a checkpoint when this
situation is detected. I haven't started to code that up, however,
wanting to first get buy-in on that.

Thanks,
James Coleman

1: https://www.postgresql.org/message-id/CAAaqYe8b2DBbooTprY4v=BiZEd9qBqVLq+FD9j617eQFjk1KvQ@mail.gmail.com