[Feature] Custom Future implementation

[TheChilliPL]

To make it possible to have many queued actions with a different algorithm with the default JavaScript FIFO event queue with Promises, a custom Future implementation could be provided.

It could have several useful features.

• Not starting automatically upon creating the object
• Using custom schedulers that could accept parameters, and thus allow more control over which actions are prioritized
• With a derived class, a CancelableFuture could be implemented, which could allow cancelling any action not yet finished.

[TheChilliPL]

Despite the unit tests passing, the implementation in 387e670 seems to crash the bot…