Implement smart algorithm of avoiding OnCooldown

I really like this approach, as it has only one caveat: memory usage.
We need to keep in memory list of all packages that our account owns, which will result of N * 32bit extra memory usage, where N is equal to number of package licenses the account owns. This results in around 16 KB extra memory usage for my 4k account.
However, apart from that there are no real caveats as checking if we own given packageID is O(1) operation, and I think that apart from this extra memory footprint there can be more benefits of having this field in future, besides, my 16 KB is extreme case, as usually nobody goes that high. Ship it!

ArchiSteamFarm/ConcurrentHashSet.cs

@@ -100,6 +100,16 @@ namespace ArchiSteamFarm {
 			}
 		}
 
+		public void TrimExcess() {
+			Lock.EnterWriteLock();
+
+			try {
+				HashSet.TrimExcess();
+			} finally {
+				Lock.ExitWriteLock();
+			}
+		}
+
 		public void Dispose() => Lock.Dispose();
 
 		public void CopyTo(T[] array, int arrayIndex) {