When creating recovery.conf outside of "repmgr standby clone",
there was no way of knowing if a replication user had been
explicitly provided with --replication-user, meaning the value
of "primary_conninfo" would be set to the "conninfo" field of the
node's upstream node record.
We'll add an extra column to store the replication user for each
node so it can be referenced at any time.
This involves adding some infrastructure to parse pg_control in a
reasonably version-independent fashion. This will probably be useful
in other contexts where we need to verify the status of a
data directory where the instance isn't running.
