Switch from PHP memcached client to CakePHP's

This is a first step to switch to storing PHP session
data into the database in a seamless way.

Setting php_admin_value[session.save_handler] in php.ini has the effect
of preventing changing session.save_handler at run time. We need to get
rid of that, so that we can use a custom session handler. Next, we’ll
be able to switch the session handler from a simple code update.

The memc.sess.key. prefix seems to be the default prefix used by PHP,
which I discovered using tcpdump.

Refs #3160.

ansible/roles/setup_php/templates/tatoeba.conf

@@ -26,7 +26,4 @@ php_admin_value[opcache.revalidate_path] = 0
 php_admin_value[opcache.validate_timestamps] = 2
 php_admin_value[opcache.max_file_size] = 0
 
-php_admin_value[session.save_handler] = memcached
-php_admin_value[session.save_path] = "localhost:11211"
-
 php_admin_value[url_rewriter.tags] = "a=href,area=hrea,frame=src,input=src,form=fakeentry"

config/app.php

@@ -144,6 +144,15 @@
             'duration' => '+1 years',
             'url' => env('CACHE_CAKEROUTES_URL', null),
         ],
+
+        'memcached' => [
+            'className' => 'Cake\Cache\Engine\MemcachedEngine',
+            'host' => 'localhost',
+            'port' => 11211,
+            'duration' => 120 * 60, // same as Session.timeout, converted to seconds
+            'prefix' => 'memc.sess.key.',
+            'serialize' => 'igbinary',
+        ],
     ],
 
     /**
@@ -404,7 +413,10 @@
      * To use database sessions, load the SQL file located at config/schema/sessions.sql
      */
     'Session' => [
-        'defaults' => 'php',
+        'defaults' => 'cache',
+        'handler' => [
+            'config' => 'memcached',
+        ],
         'timeout' => 120,
     ],
 ];