Get a better definition of what is in each protocol version

[justinjdickow]

Right now we're about to release protocol version 4 in each of the libraries and on core but they don't have a full heartbeat or video streaming implementation (on the mobile side). This will cause incompatibility if, for example, a version 3 sdl core starts sending heart beats and doesn't get any response from a version 4 library.

One of our proposals is to rearrange the features that are supported in each version of the protocol. For example, if heartbeat is not supported in the next release of iOS and Android, we might have to move it's support to V 4 instead of V 3.

We also have an issue that Apple changed how canOpenUrl works and so we might have to remove the majority of features in V4.