Re: Large files for relations

Rebased.  I had intended to try to get this into v17, but a couple of
unresolved problems came up while rebasing over the new incremental
backup stuff.  You snooze, you lose.  Hopefully we can sort these out
in time for the next commitfest:

* should pg_combinebasebackup read the control file to fetch the segment size?
* hunt for other segment-size related problems that may be lurking in
new incremental backup stuff
* basebackup_incremental.c wants to use memory in proportion to
segment size, which looks like a problem, and I wrote about that in a
new thread[1]

[1]
https://www.postgresql.org/message-id/flat/CA%2BhUKG%2B2hZ0sBztPW4mkLfng0qfkNtAHFUfxOMLizJ0BPmi5%2Bg%40mail.gmail.com