[FileCache] Use serialize instead of json_encode

json_encode causes high memory footprint on large input data,
where serialize is less problematic.

Example: When using AcrimedBridge items contain pictures in
raw format (entire picture) which leads to a file size of about
2MB using serialize. json_encode will allocate about 98MB of
memory for encoding, causing memory exhausion errors (PHP
allows for 128MB of memory by default)

caches/FileCache.php

@@ -8,11 +8,13 @@ class FileCache implements CacheInterface {
 	protected $param;
 
 	public function loadData(){
-		return json_decode(file_get_contents($this->getCacheFile()), true);
+		return unserialize(file_get_contents($this->getCacheFile()));
 	}
 
 	public function saveData($datas){
-		$writeStream = file_put_contents($this->getCacheFile(), json_encode($datas, JSON_PRETTY_PRINT));
+		// Notice: We use plain serialize() here to reduce memory footprint on
+		// large input data.
+		$writeStream = file_put_contents($this->getCacheFile(), serialize($datas));
 
 		if($writeStream === false) {
 			throw new \Exception("Cannot write the cache... Do you have the right permissions ?");
@@ -110,6 +112,8 @@ class FileCache implements CacheInterface {
 			throw new \Exception('Call "setParameters" first!');
 		}
 
-		return hash('md5', http_build_query($this->param)) . '.cache';
+		// Change character when making incompatible changes to prevent loading
+		// errors due to incompatible file contents         \|/
+		return hash('md5', http_build_query($this->param) . 'A') . '.cache';
 	}
 }