This commit improves the performance of page-level, or differential
backup, by not having to scan anymore all the pages of a relation file,
something that can be very long on large data sets, but by scanning the
list of blocks changed by WAL records since the last full or differential
backup.
As a restriction and to avoid potential data corruption should hint-bit
updates occur on a page, backups can only be taken from a server that has
wal_log_hints or data checksums enabled.
Base patch by Yury Zhuravlev, heavily modified by me.
Performance of compression is quite questionable on many objects like
that and makes the routines aimed at managing file copy, backup and restore
more complicated than they should be.
This commit results in a largely simplified code in data.c, which will
be helpful when integrating differential backup using WAL file lookup.
This new facility has the advantage on not relying on static data when
generating the tests making the whole facility more robust. This is
basically taken from the upstream project pg_rman and adapted for the
sake of this pet project, so most of the credit go to Kyotaro Horiguchi
and Amit Langote regarding this facility. However I have adapted a bunch
of things and fixed a lot of redundancy and code duplication.
