Always store state in an Arc (#1270)

* Add extension and state benchmarks

* wip

* Arc the state everywhere

* don't require `S: Clone`

* fix example

examples/versioning/src/main.rs

@@ -51,7 +51,7 @@ enum Version {
 impl<S, B> FromRequest<S, B> for Version
 where
     B: Send,
-    S: Send,
+    S: Send + Sync,
 {
     type Rejection = Response;