All the ERROR_* fields are removed in favor of a more simple layer
made of ERROR, FATAL, PANIC. The two last ones are not actually used
yet, thought there should be some code paths that would need more
polishing on this matter.
The error message emitted before leaving should be fine to let the
user know what is happening.
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.
