Remove Deprecated Exclusive Backup Mode

Hackers,

I propose we remove the deprecated exclusive backup mode of 
pg_start_backup()/pg_stop_backup() for Postgres 12.

The exclusive backup mode has a serious known issue.  If Postgres 
terminates ungracefully during a backup (due to hardware, kernel, 
Postgres issues, etc.) then Postgres may refuse to restart.

The reason is that the backup_label file will usually reference a 
checkpoint LSN that is older than the WAL available in pg_wal.  Postgres 
does emit a helpful error message while PANIC'ing but that's cold 
comfort to an admin who must manually intervene to get their cluster 
operational again.

The deprecated exclusive mode promises to make a difficult problem 
simple but doesn't live up to that promise. That's why it was replaced 
externally in 9.6 and why pg_basebackup has not used exclusive backups 
since it was introduced in 9.2.

Non-exclusive backups have been available since 9.6 and several 
third-party solutions support this mode, in addition to pg_basebackup.

The recently introduced recovery changes will break current automated 
solutions so this seems like a good opportunity to make improvements on 
the backup side as well.

I'll submit a patch for the 2019-01 commitfest.

Regards,
-- 
-David
david@pgmasters.net