AuthCallback and InstanceStore Refactoring and documentation

[ben-xD]

This work improves authCallback documentation so users can test locally more easily.

It also simplifies the code further, continuing the work of #231.

Mainly,

    void setRealtimeToken(long handle, Object tokenDetails) {
        _realtimeTokenData.put(handle, tokenDetails);
    }

    Object getRealtimeToken(long handle) {
        Object token = _realtimeTokenData.get(handle);
        _realtimeTokenData.remove(handle);
        return token;
    }

There is no need to save tokenDetails in AblyLibrary, it can be returned immediately in the callback.

Once there, I wanted to improve the naming of AblyLibrary. It only does one thing: store instances, and so it's been renamed to AblyInstanceStore.

I also move the logic of creating clients outside of AblyInstanceStore. This paves the way for futher improvements: moving usage of AblyInstanceStore into the Codec as opposed to the method call handlers. Therefore, in the future, we might not need to deal with "handles", since we could deserialize the handle into the instance in the codec. This helps a transition to Swift too, since things are more decoupled.

There's lots of refactoring going on in this file. There is no logic changes, except for the one above.

I'm sorry for making this PR so big.

[lukasz-szyszkowski]

Please re-check your files and add newline at the end of each modified file.

[ikbalkaya]

Light review warning - I skimmed through changes as this is a significantly big PR. One think I realised was use of lots of statics. As this is a refactor work I assume there is a good reason for them, but I have to say that use of lots of statics makes me anxious.