Unify PostgresNode's new() and get_new_node() methods

There is only one constructor now for PostgresNode, with the idiomatic
name 'new'. The method is not exported by the class, and must be called
as "PostgresNode->new('name',[args])". All the TAP tests that use
PostgresNode are modified accordingly. Third party scripts will need
adjusting, which is a fairly mechanical process (I just used a sed
script).

src/test/authentication/t/001_password.pl

@@ -62,7 +62,7 @@ sub test_role
 }
 
 # Initialize primary node
-my $node = get_new_node('primary');
+my $node = PostgresNode->new('primary');
 $node->init;
 $node->append_conf('postgresql.conf', "log_connections = on\n");
 $node->start;

src/test/authentication/t/002_saslprep.pl

@@ -62,7 +62,7 @@ sub test_login
 
 # Initialize primary node. Force UTF-8 encoding, so that we can use non-ASCII
 # characters in the passwords below.
-my $node = get_new_node('primary');
+my $node = PostgresNode->new('primary');
 $node->init(extra => [ '--locale=C', '--encoding=UTF8' ]);
 $node->start;