postgres: Changes to work with version 12

In postgres 12, recovery.conf is removed in favor of normal config options
plus a signal file.

See https://www.2ndquadrant.com/en/blog/replication-configuration-changes-in-postgresql-12/
for a good rundown.

postgres/standby_setup.sh

@@ -8,11 +8,14 @@ if [ ! -s "$PGDATA/PG_VERSION" ]; then
 	# get a binary backup of the database on $MASTER_NODE
 	pg_basebackup -d "host=$MASTER_NODE password='$REPLICATION_PASSWORD' port=5432 user=$REPLICATION_USER" -D ${PGDATA} -vP
 
-	cat > ${PGDATA}/recovery.conf <<-EOF
-	standby_mode = on
+	# indicate postgres should start in hot standby mode
+	touch "$PGDATA/standby.signal"
+
+	# write replication settings to config file
+	cat >> ${PGDATA}/postgresql.conf <<-EOF
 	primary_conninfo = 'host=$MASTER_NODE password=\\'$REPLICATION_PASSWORD\\' port=5432 user=$REPLICATION_USER'
 	# touch this file to promote this node to master
-	trigger_file = '/tmp/touch_to_promote_to_master'
+	promote_trigger_file = '/tmp/touch_to_promote_to_master'
 	EOF
 
 	chown postgres. ${PGDATA} -R