Fix DartVM not exiting even when closing the youtube client.

Closes #61

lib/src/reverse_engineering/responses/player_source.dart

@@ -112,20 +112,51 @@ class PlayerSource {
   ///
   static Future<PlayerSource> get(
       YoutubeHttpClient httpClient, String url) async {
-    if (_cache[url] == null) {
-      Timer(const Duration(minutes: 10), () {
-        _cache[url] = null;
-      });
-
-      return _cache[url] = await retry(() async {
+    if (_cache[url]?.expired ?? true) {
+      var val = await retry(() async {
         var raw = await httpClient.getString(url);
         return PlayerSource.parse(raw);
       });
+      if (_cache[url] == null) {
+        _cache[url] = _CachedValue(val);
+      } else {
+        _cache[url].update(val);
+      }
     }
-    return _cache[url];
+    return _cache[url].value;
   }
 
-  static final Map<String, PlayerSource> _cache = {};
+  static final Map<String, _CachedValue<PlayerSource>> _cache = {};
+}
+
+class _CachedValue<T> {
+  T _value;
+  int expireTime;
+  final int cacheTime;
+
+  T get value {
+    if (expired) {
+      throw StateError('Value $value is expired!');
+    }
+    return _value;
+  }
+
+  bool get expired {
+    final now = DateTime.now().millisecondsSinceEpoch;
+    return now > expireTime;
+  }
+
+  set value(T other) => _value = other;
+
+  _CachedValue(this._value, [this.expireTime, this.cacheTime = 600000]) {
+    expireTime ??= DateTime.now().millisecondsSinceEpoch + cacheTime;
+  }
+
+  void update(T newValue) {
+    var now = DateTime.now().millisecondsSinceEpoch;
+    expireTime = now + cacheTime;
+    value = newValue;
+  }
 }
 
 extension on String {