[RFC] Incremental backup v2: add backup profile to base backup

Hi Hackers,

I've updated the wiki page
https://wiki.postgresql.org/wiki/Incremental_backup following the result
of discussion on hackers.

Compared to first version, we switched from a timestamp+checksum based
approach to one based on LSN.

This patch adds an option to pg_basebackup and to replication protocol
BASE_BACKUP command to generate a backup_profile file. It is almost
useless by itself, but it is the foundation on which we will build the
file based incremental backup (and hopefully a block based incremental
backup after it).

Any comment will be appreciated. In particular I'd appreciate comments
on correctness of relnode files detection and LSN extraction code.

Regards,
Marco

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Marco Nenciarini - 2ndQuadrant Italy
PostgreSQL Training, Services and Support
marco(dot)nenciarini(at)2ndQuadrant(dot)it | www.2ndQuadrant.it