Add support for Memcached/Improve database cache

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

src/Core/Cache/MemcacheCacheDriver.php

@@ -0,0 +1,77 @@
+<?php
+
+namespace Friendica\Core\Cache;
+
+use Friendica\BaseObject;
+use Friendica\Core\Cache;
+
+/**
+ * Memcache Cache Driver
+ *
+ * @author Hypolite Petovan <mrpetovan@gmail.com>
+ */
+class MemcacheCacheDriver extends BaseObject implements ICacheDriver
+{
+	/**
+	 * @var Memcache
+	 */
+	private $memcache;
+
+	public function __construct($memcache_host, $memcache_port)
+	{
+		if (!class_exists('Memcache', false)) {
+			throw new \Exception('Memcache class isn\'t available');
+		}
+
+		$this->memcache = new \Memcache();
+
+		if (!$this->memcache->connect($memcache_host, $memcache_port)) {
+			throw new \Exception('Expected Memcache server at ' . $memcache_host . ':' . $memcache_port . ' isn\'t available');
+		}
+	}
+
+	public function get($key)
+	{
+		$return = null;
+
+		// We fetch with the hostname as key to avoid problems with other applications
+		$cached = $this->memcache->get(self::getApp()->get_hostname() . ':' . $key);
+
+		// @see http://php.net/manual/en/memcache.get.php#84275
+		if (is_bool($cached) || is_double($cached) || is_long($cached)) {
+			return $return;
+		}
+
+		$value = @unserialize($cached);
+
+		// Only return a value if the serialized value is valid.
+		// We also check if the db entry is a serialized
+		// boolean 'false' value (which we want to return).
+		if ($cached === serialize(false) || $value !== false) {
+			$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->memcache->set(
+			self::getApp()->get_hostname() . ":" . $key,
+			serialize($value),
+			MEMCACHE_COMPRESSED,
+			Cache::duration($duration)
+		);
+	}
+
+	public function delete($key)
+	{
+		return $this->memcache->delete($key);
+	}
+
+	public function clear()
+	{
+		return true;
+	}
+}