#100 Global backup and restore

[Koichi Suzuki]

Now XC leverages vanilla PostgreSQL's archive backup and PITR with additional Barrier feature. Because each node independently archives their WAL segment, we need a means to find out what BARRIER is available for specific restoration.
Additionally, gtm need its backup which can be used with specific BARRIER restoration. Maybe CREATE BARRIER should be reported to GTM and GTM should have its status snapshot with BARRIER ID to make cluster-wide consistent restoration.