Refactor `stream::Shared` to use internal mutability

[thomaseizinger]

Currently, we hold a stream::Shared within the stream and one in Active. This is shared by being wrapped in Arc<Mutex>.

It would be slightly more ergonomic for both usage sites if the Arc<Mutex> would be inside stream::Shared.

[akaladarshi]

Hey @mxinden

Is this issue still available for pickup?

[dariusc93]

Yes it is @akaladarshi :)

[akaladarshi]

@dariusc93 Can you assign it to me?.

Also just to get the full idea: We need to put Arc<Mutex> inside stream::Shared, so we don't have to use Arc<Mutex<Shared>> everywhere else.