Turn Option<AgentState> into AgentState (#149)

`Some(AgentState)` seem to be assumed pretty much everywhere. I could not
find any test or piece of code hinting at how `None` should be interpreted,
or even see how a state of `None` could even be constructed.

Co-authored-by: Ulrik <ulrikm@spotify.com>

src/test/agent_test.rs

@@ -78,8 +78,7 @@ fn connection_reuse() {
     resp.into_string().unwrap();
 
     {
-        let mut guard_state = agent.state.lock().unwrap();
-        let mut state = guard_state.take().unwrap();
+        let mut state = agent.state.lock().unwrap();
         assert!(state.pool().len() > 0);
     }