Make hub_connection always a shared_ptr

[BrennanConroy]

We talked about replacing the hub_connection_impl and making hub_connection contain all the logic. However, that would require including a lot of internal types in the public header and all sorts of ugliness.

[halter73]

Weren't we going to turn hub_connection_impl into hub_connection? Otherwise, we're essentially returning a share_ptr<shared_ptr<real_hub_connection> >. While not necessarily wrong, I don't see the point.

[BrennanConroy]

I gave a little explanation in the initial PR comment

[halter73]

We talked about replacing the hub_connection_impl and making hub_connection contain all the logic. However, that would require including a lot of internal types in the public header and all sorts of ugliness.

There has to be some way to get rid of the redundant share_ptr without exposing internal types, no?

[jwittner]

This change would be great @BrennanConroy. We've recently got this client working in Unreal, but the current API makes it difficult to store a hub_connection in our classes (there's no default or copy constructors). We had to add a default constructor to the class in order to do so.

This change looks like the right kind of solution.

[TobyGilbert-ms]

Looks like there's a move constructor available for hub_connection which allows us to use a shared pointer. But returning a shared pointer directly from the builder would be useful.