Remove `Sync` bounds about httprouter-rs

[ibraheemdev]

Wrapping the router in a mutex would mean requiring mutual exclusion during the creation of a response future, which is not something I want to do. Is there a reason the Sync bound is a problem?

[malobre]

In my experience, future are often !Sync.
I use futures::future::BoxFuture quite a lot and it isn't Sync, the same goes for https://github.com/dtolnay/async-trait which expands to !Sync futures.