accesslib: fix SQL CAST()s to work on MySQL and PostgreSQL

CAST() target types aren't very portable. Use DECIMAL which works
for MySQL and Pg.

DECIMAL does seem to be supported in Oracle - but the syntax seems
different. We may still need a compat function.

lib/accesslib.php

@@ -4586,7 +4586,7 @@ function get_dirty_contexts($time) {
     $sql = "SELECT name, value 
             FROM {$CFG->prefix}config_plugins
             WHERE plugin='accesslib/dirtycontexts'
-                  AND CAST(value AS integer) > $time";
+                  AND CAST(value AS DECIMAL) > $time";
     if ($ctx = get_records_sql($sql)) {
         return $ctx;
     }
@@ -4620,7 +4620,7 @@ function cleanup_dirty_contexts() {
     global $CFG;
     
     $sql = "plugin='accesslib/dirtycontexts' AND
-                  CAST(value to integer) < " . time() - $CFG->sessiontimeout;
+                  CAST(value AS DECIMAL) < " . time() - $CFG->sessiontimeout;
     delete_records_select('config_plugins', $sql);
 }