[WIP] Add extension for wal-g incremental backups

[debebantur]

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[Alena0704]

Hi Stepan, thanks for working on this!
A few questions on the design before I look at the code.

1. Why an extension rather than core?
2. How is the record made crash-safe? VACUUM can't run inside a transaction block, so the compaction and the INSERT into possible_vacuum_list are two separate commits. If the server crashes in between, the segfile is already rewritten but the vacuum is not recorded and it will corrupt an incremental backup.
3. Why a new table instead of pg_aoseg? Did you consider deriving info from existing catalog/stats instead of a new table - e.g. pg_stat_all_tables.last_vacuum, or the eof/modcount in pg_aoseg?

Also, I'm worried that the module that installs a utility hook probably shouldn't be installable by a non-superuser.