MDL-60915 core_dml: Change behavior to disabled by default

In stables it was decided to keep the current behavior
(not buffering/cursors) so nothing breaks/changes suddenly.

So this commit just disables the default buffer size used in
Moodle 3.5 and up (100000), defaulting to zero (feature disabled).

Still the feature can be enabled via dboptions and it's docummented
in the config-dist file.

lib/dml/pgsql_native_moodle_database.php

@@ -48,9 +48,6 @@ class pgsql_native_moodle_database extends moodle_database {
     /** @var int Number of cursors used (for constructing a unique ID) */
     protected $cursorcount = 0;
 
-    /** @var int Default number of rows to fetch at a time when using recordsets with cursors */
-    const DEFAULT_FETCH_BUFFER_SIZE = 100000;
-
     /**
      * Detects if all needed PHP stuff installed.
      * Note: can be used before connect()
@@ -782,7 +779,7 @@ class pgsql_native_moodle_database extends moodle_database {
         if (array_key_exists('fetchbuffersize', $this->dboptions)) {
             return (int)$this->dboptions['fetchbuffersize'];
         } else {
-            return self::DEFAULT_FETCH_BUFFER_SIZE;
+            return 0; // Disabled by default.
         }
     }