offchain: Upgrade hyper to 0.13, which uses tokio 0.2 (#4860)

* service: Don't depend on tokio-executor

Seems to be a leftover dependency that's not used anymore.

* offchain: Upgrade hyper to 0.13, which uses tokio 0.2

* offchain: Adapt HTTP tests to Tokio 0.2

* network: Don't transitively include tokio 0.2 in WASM

1) We don't specifically depend on Tokio codec impls
2) Conflating features in Cargo means that enabling Tokio runtime
in the native environment will also do so in WASM, where it's
obviously not implemented and causes a compilation error.

* grafana-data-source: Pull hyper/tokio only in native environment