MDL-70055 pgsql DML: Support large number of SQL-IN parameters.

This patch adds a solution for Postgres that allows overriding IN
statement limit of 16bit (65535 values) by wrapping items in VALUES
list. This only takes place in number of items exceeds 65535, otherwise
parent class get_in_or_equal() function is used.

lib/dml/moodle_database.php

@@ -137,7 +137,7 @@ abstract class moodle_database {
     /**
      * @var int internal temporary variable used to guarantee unique parameters in each request. Its used by {@link get_in_or_equal()}.
      */
-    private $inorequaluniqueindex = 1;
+    protected $inorequaluniqueindex = 1;
 
     /**
      * @var boolean variable use to temporarily disable logging.