feat(sharded): add an option for dynamic private channels

[MartinKolarik]

As an alternative solution for #524, this:

• adds a new option dynamicPrivateChannels, which allows creating separate channels even for private rooms; defaults to false so there's no change in default behavior,
• includes the fix from fix(sharded): private room broadcast in dynamic mode #525 which is needed when the option is set to false

[darrachequesne]

Actually, I'm not sure that this is a good idea, as it could indicate that calling io.to(specificSocketId).emit() is recommended.

@MartinKolarik thoughts?

[MartinKolarik]

@darrachequesne I think it makes sense if it's documented in regards to socketInstance.emit() as that method is public and not discouraged, as I understand (unlike using the ID directly). Maybe with a note that it's useful in some scenarios where you have lots of 1:1 communication and understand what you're doing. In our specific case, this simple change resulted in 70% - 80% overall redis load reduction.

[darrachequesne]

Thanks 👍