Add system identifier to backup manifest

Hi All,

With the attached patch, the backup manifest will have a new key item as
"System-Identifier" 64-bit integer whose value is derived from pg_control while
generating it, and the manifest version bumps to 2.

This helps to identify the correct database server and/or backup for the
subsequent backup operations.  pg_verifybackup validates the manifest system
identifier against the backup control file and fails if they don’t match.
Similarly, pg_basebackup increment backup will fail if the manifest system
identifier does not match with the server system identifier.  The
pg_combinebackup is already a bit smarter -- checks the system identifier from
the pg_control of all the backups, with this patch the manifest system
identifier also validated. 

For backward compatibility, the manifest system identifier validation will be
skipped for version 1.