Connector::connect already used &self, and so would require
synchronization to be handled per connector anyway. Adding Sync to the
Client allows users to setup config for a Client once, such as using a
single connection Pool, and then making requests across multiple
threads.
Closes#254
BREAKING CHANGE: Connectors and Protocols passed to the `Client` must
now also have a `Sync` bounds, but this shouldn't break default usage.
