MDL-65292 style: Fix all the function declaration ordering

This has been generated running the following Sniff,
part of the Moodle's CodeSniffer standard:
- PSR2.Methods.MethodDeclaration

It just ensures all the function declarations have
the correct order for:
- abstract and final.
- visibility (public, protected, private).
- static.

So, all the lines modified by this commit are function declarations
and the only changes are in the positions of those keywords.

lib/classes/session/handler.php

@@ -91,7 +91,7 @@ abstract class handler {
     /**
      * Init session handler.
      */
-    public abstract function init();
+    abstract public function init();
 
     /**
      * Check the backend contains data for this session id.
@@ -101,17 +101,17 @@ abstract class handler {
      * @param string $sid
      * @return bool true if session found.
      */
-    public abstract function session_exists($sid);
+    abstract public function session_exists($sid);
 
     /**
      * Kill all active sessions, the core sessions table is
      * purged afterwards.
      */
-    public abstract function kill_all_sessions();
+    abstract public function kill_all_sessions();
 
     /**
      * Kill one session, the session record is removed afterwards.
      * @param string $sid
      */
-    public abstract function kill_session($sid);
+    abstract public function kill_session($sid);
 }