Add sync on messagesender

[DaanA32]

Add Sync to MessageSender to align it with documentation:

/// A context for sending messages to a component or the runtime.
///
/// It can be used in a multi-threaded environment (implements `Sync` and
/// `Send`). Additionally, it can send messages to the runtime, in particular
/// it's used to gracefully stop a running [`App`](struct.App.html).
#[derive(Debug)]
pub struct ComponentLink<ComponentT> {
     ...
}