backup tools ought to ensure created backups are durable

Hi,

As pointed out in
http://www.postgresql.org/message-id/20160327232509.v5wgac5vskusedin@awork2.anarazel.de
our backup tools (i.e. pg_basebackup, pg_dump[all]), currently don't
make any efforts to ensure their output is durable.

I think for backup tools of possibly critical data, that's pretty much
unaceptable.

There's cases where we can't ensure durability (i.e. pg_dump | gzip >
file), but it's out of our hands in that case.

Greetings,

Andres Freund