Move next_stream_idto ConnectionContext

[TimEvens]

Currently next_stream_id is at the transport class scope. This is fine for clients, but it's not really ideal for servers. Moving this to ConnectionContext will make this connection specific, which is what it should be.

When moving it should be renamed to last_stream_id since it's really the last stream number assigned, not the next one available. This is used to feed it back into make_stream_id()