Allow non-empty connection pools to be dropped (#583)

This breaks a reference cycle between PoolReturner and Agent that was
causing Agents (and their contained ConnectionPool) to never be dropped
so long as there was any stream in the ConnectionPool. This cause
sockets to leak over time, particularly when the convenience functions
ureq::get(), ureq::post(), etc were used, since those functions create
a new Agent each time.

src/agent.rs

@@ -238,6 +238,10 @@ impl Agent {
     pub fn cookie_store(&self) -> CookieStoreGuard<'_> {
         self.state.cookie_tin.read_lock()
     }
+
+    pub(crate) fn weak_state(&self) -> std::sync::Weak<AgentState> {
+        Arc::downgrade(&self.state)
+    }
 }
 
 const DEFAULT_MAX_IDLE_CONNECTIONS: usize = 100;