Obfs4 features

[jmwample]

Add the rest of the missing features for obfs4 and solidify the implementation style.

• Add IAT delays sampled from drbg based on the seed.
• Add IAT length sampling.
• shift from using AsyncRead / AsyncWrite to using Sink / Stream which seems to makes more sense

[jmwample]

Can RefCell<T>, Sink::boxed(...), Stream::boxed(...), save us from having to use the 'static lifetime?

And can AsRef<dyn Sink<...>>, and AsRef<dyn Stream<...>> allow us to expose the inner Sink and Stream trait objects by reference?

That way they just do the right thing internally and the O4Stream and Obfs4Stream types just keep track of everything.