Refactoring searchClientById

Refactoring searchClientById

searchClientById was scanning through all open sockets to find the right one (which is inefficient if we have many).
Instead, I created a new Map that directly maps ids to sockets.
Furthermore, this solves a long-standing issue (when a socket is disconnected, we cannot find it anymore in the sockets list but it is still available in the disconnect callback from the map)
As a result, we should not have any remaining circles any more.