- `Client::new()` no longer needs a `Handle`, and instead makes use of
tokio's implicit default.
- Changed `Client::configure()` to `Client::builder()`.
- `Builder` is a by-ref builder, since all configuration is now
cloneable pieces.
BREAKING CHANGE: `Client:new(&handle)` and `Client::configure()` are now
`Client::new()` and `Client::builder()`.