Add a shared character state

[jon-zu]

• Create a shared state for each session behind a lock, which book-keeps changes
• Expose the shared state via the Session handle
• Upon updating the state, notify the Session via an Update message

Further thoughts:
Deadlocks and other problems should be avoided per-design. As in not exposing the lock directly and just having functions to access and update the state. For example instead of getting a mutable reference to the current stats we should allow add/sub stats:

state.sub_stats(CharStats {
 hp: 50,
 mp: 5,
 ..Default::default()
});

instead of exposing the lock and doing It by hand:

state.lock().hp -= 50;
state.lock().mp -= 5;

Which is also prone to overflows.