MDL-37683 cache: siteidentifier is now included in the keys

Conflicts:
	lib/db/upgrade.php
	lib/moodlelib.php
	version.php

cache/classes/config.php

@@ -71,6 +71,12 @@ class cache_config {
      */
     protected $configlocks = array();
 
+    /**
+     * The site identifier used when the cache config was last saved.
+     * @var string
+     */
+    protected $siteidentifier = null;
+
     /**
      * Please use cache_config::instance to get an instance of the cache config that is ready to be used.
      */
@@ -139,6 +145,12 @@ class cache_config {
         $this->configdefinitionmappings = array();
         $this->configlockmappings = array();
 
+        $siteidentifier = 'unknown';
+        if (array_key_exists('siteidentifier', $configuration)) {
+            $siteidentifier = $configuration['siteidentifier'];
+        }
+        $this->siteidentifier = $siteidentifier;
+
         // Filter the lock instances.
         $defaultlock = null;
         foreach ($configuration['locks'] as $conf) {
@@ -271,6 +283,14 @@ class cache_config {
         return true;
     }
 
+    /**
+     * Returns the site identifier used by the cache API.
+     * @return string
+     */
+    public function get_site_identifier() {
+        return $this->siteidentifier;
+    }
+
     /**
      * Includes the configuration file and makes sure it contains the expected bits.
      *