fix: fix race condition with cache causing ytdl-core to hang

sometimes when clearing the ytdl-core internal cache, the app would
still hang, waiting for any entries in the cache to expire.

lib/cache.js

@@ -5,6 +5,9 @@ module.exports = class Cache extends Map {
     this.timeout = timeout;
   }
   set(key, value) {
+    if (this.has(key)) {
+      clearTimeout(super.get(key).tid);
+    }
     super.set(key, {
       tid: setTimeout(this.delete.bind(this, key), this.timeout),
       value,
@@ -17,11 +20,11 @@ module.exports = class Cache extends Map {
     }
     return null;
   }
-  async getOrSet(key, fn) {
+  getOrSet(key, fn) {
     if (this.has(key)) {
       return this.get(key);
     } else {
-      let value = await fn();
+      let value = fn();
       this.set(key, value);
       return value;
     }