Catchup from delta-backup

[x4m]

I'll create an issue for this, hope one day we will implement it. Does not seem a big chunk of work and testing. Though I expect only hard-boiled DBA want something like this.

Currently, delta backups are applied on top of other backups. Either base backup or other delta backup.
Delta backup Y from backup X contains all blocks changed since the start of backups X. Also, Y will contain all files WAL-G cannot track blockwise: SLRUs, configs, FSMs, VMs etc.

When a replica is falling behind the primary we can use information from delta-backup to jump through the WAL history: we know blocks numbers that need to be updated.
Here I propose to implement command like wal-g catchup PATH --from-lsn LSN.
PATH should contain PGDATA shut down in recovery somewhere after LSN. WAL-G will look through delta backups and try to restore on most recent point in the history of the backup.

O has somewhat similar functionality in RMAN.

We can read LSN from pg_control, but this seems too intrusive.

[g0djan]

Commited