Re-enable the client's custom executor support

[seanmonstar]

As part of the std::future update, the client's custom executor code is unimplemented!. We should get it working again.

Some notes:

• The Executor trait now requires &mut self, so the previous Arc<dyn Executor> won't work. Most executors already have synchronized queuing internally, so it'd be a shame to use a mutex here.
• It'd be even better to make the executor a generic, so that we could remove the bounds requiring the Connector and Body types to be thread-safe. This would need a bit of work, converting most of the Clients internal futures to not use combinators, but to be nameable types. Then, we could have some E: ClientExec trait similar to the server side.

[seanmonstar]

The first option was taken for now, requiring for<'a> &'a E: Executor, which most executors can provide easily.