Introduce subscription ID

[FZambia]

At moment Centrifuge does not allow to subscribe on the same channel twice inside single connection. This is ok for most use cases but leads to difficulties and non very clear API restrictions on client side (users can only create one subscription to the same channel).

I believe introducing incremental subscription ID should solve this.

[FZambia]

After considering a bit - not sure this is simple without loosing performance at moment. Because subscription ID should be included to every Publication. Maybe the solution should be on client side – will continue thinking on this. Closing for now.