No singleton: ChannelContext, ChannelBuilder and channel cache

[Klaim]

Both ChannelBuilder and channel cache are now fusioned into ChannelContext which is now responsible for storing and providing Channel instances, and is not a singleton anymore.

[Klaim]

Cancelling the build because this is definitely not ready yet.
I'm having trouble making sens of the usage of make_channel(...) as it seems to often be used not really to create channel but to obtain related names/values. As I'm trying to not break the meaning of the code, for now I'm avoiding blindly adding parameters where it might not really make sense.
So this is a work-in-progress.

[Klaim]

Note that a lot of cases where ChannelContext is added as a parameter of a function is because that function will use .make_channel just to "parse" a channel value and extract some of it's information (like it's name). We had some discussions about this and some potential plans, I did NOT address this here and just made the ChannelContext available where it's need. Future refactoring might remove that parameters where I added it here.

[AntoinePrv]

Well done 🎉 I could not really follow everywhere, but I have not seen anything shocking.
Let's merge this before it starts conflicting with everything.

[AntoinePrv]

Suggested change

	std::cout << q.find(query).groupby("name").json(pool.channel_context()).dump(4);
	std::cout << q.find(query).groupby("name").json(channel_context).dump(4);

[AntoinePrv]

And similar below.