Channels visibility improvements

[QuintinWillison]

The quick win was making the channels field on realtime instances final - preventing that reference from being trampled.

The larger change was a bit more complex, involving (both) the Channels interfaces we have. They were classes but are now interfaces with private implementations, solving the following accessibility issues:

1. Constructor: It should never have been accessible, however it was also causing an issue for Kotlin developers because the lack of a new keyword makes it less obvious that you're creating an instance of an inner class.
2. Underlying map implementation: Allowing anybody in possession of a Channels reference to mutate the channels map without the Ably implementation knowing.
3. Internal methods: Called by the connection manager but could equally be called by anybody in possession of a Channels reference without the connection manager knowing.

Aside

I wanted to push further and deprecate the channels fields on both AblyRealtime and AblyBase, however because AblyRealtime is a subclass of AblyBase it was not as easy as just creating a getChannels() method. The problem is that fields can shadow superclass fields and change their type at the same time, even to an incompatible type, but methods can't. And the Channels returned by base are of a completely different class to the Channels returned by realtime (due to the fact that we also have two forks of ChannelBase upon which each is ultimately reliant!).

[QuintinWillison]

Yes, kids, it's real. Always, always, always run tests before you push!

[QuintinWillison]

Sorry, @paddybyers, I've broken all kinds of tests. Shall fix those and then re-add my request for review once done.

[paddybyers]

LGTM, thanks