Upgrade reqwest and change timeout to read_timeout (#1014)

- Upgrade reqwest to 0.13.2 and adapt to API changes
- Remove deprecated `backoff_exponent()` from ExponentialBackoff builder
- Adapt `reqwest_middleware::RequestBuilder` API changes by using explicit JSON serialization
- Fix all clippy warnings (dead_code, clone_on_copy, manual_is_multiple_of, lifetime syntax)