Add support for Memcached/Improve database cache

- Create Cache Driver interface
- Update cache table fields
- Add CacheSessionHandler

src/Core/Cache/MemcachedCacheDriver.php

@@ -0,0 +1,66 @@
+<?php
+
+namespace Friendica\Core\Cache;
+
+use Friendica\BaseObject;
+use Friendica\Core\Cache;
+
+/**
+ * Memcached Cache Driver
+ *
+ * @author Hypolite Petovan <mrpetovan@gmail.com>
+ */
+class MemcachedCacheDriver extends BaseObject implements ICacheDriver
+{
+	/**
+	 * @var Memcached
+	 */
+	private $memcached;
+
+	public function __construct($memcached_host, $memcached_port)
+	{
+		if (!class_exists('Memcached', false)) {
+			throw new \Exception('Memcached class isn\'t available');
+		}
+
+		$this->memcached = new \Memcached();
+
+		if (!$this->memcached->addServer($memcached_host, $memcached_port)) {
+			throw new \Exception('Expected Memcached server at ' . $memcached_host . ':' . $memcached_port . ' isn\'t available');
+		}
+	}
+
+	public function get($key)
+	{
+		$return = null;
+
+		// We fetch with the hostname as key to avoid problems with other applications
+		$value = $this->memcached->get(self::getApp()->get_hostname() . ':' . $key);
+
+		if ($this->memcached->getResultCode() === \Memcached::RES_SUCCESS) {
+			$return = $value;
+		}
+
+		return $return;
+	}
+
+	public function set($key, $value, $duration = Cache::MONTH)
+	{
+		// We store with the hostname as key to avoid problems with other applications
+		return $this->memcached->set(
+			self::getApp()->get_hostname() . ":" . $key,
+			$value,
+			Cache::duration($duration)
+		);
+	}
+
+	public function delete($key)
+	{
+		return $this->memcached->delete($key);
+	}
+
+	public function clear()
+	{
+		return true;
+	}
+}