Add hyper 0.12 based HTTP transport implementation

[faern]

This is trying to move away from the now old tokio_core and hyper 0.11.

I'm planning on replacing jsonrpc_client_http completely with this new hyper 0.12 based transport. But as a transition while it's being developed I opted to keep both until we know the new one is usable. So the name newhttp is not something that I ever mean to publish or anything.

This implementation does not support custom headers. But for our use case we should not need that any more. We previously needed it for host headers when we connected directly to IPs. But with the new DNS cache solution we should be able to connect directly to our domains and no special hacks for SNI or host header insertion should be needed.

This transport is also a lot simpler. It does not contain any "client creator" code. And no helpers for creating hyper clients. The user just have to hand us any hyper::Client and we'll work with that directly.

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This change is 

[faern]

Reviewable status: 0 of 12 files reviewed, 1 unresolved discussion

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newhttp/examples/simple.rs, line 14 at r1 (raw file):

    let jsonrpc_transport = jsonrpc_client_newhttp::HttpTransport::new(client, None);

    let uri = "https://api.mullvad.net/rpc/".parse::<Uri>().unwrap();

There is a bug somewhere, and I'm not able to track it down. If you change this URI into https://api.mullvad.net/rpc (remove the last /) then our server will return a 301 status code. This will result in this example binary to quit with:

[2019-02-27T13:41:05Z TRACE jsonrpc_client_newhttp] Sending request to https://api.mullvad.net/rpc
[2019-02-27T13:41:05Z DEBUG jsonrpc_client_newhttp] Got response with status 301 Moved Permanently
Error processing request: RPC Client already shut down

The entire transport is not supposed to exit just because it receives a 302 response. I have tried adding logging here and there in newhttp and I can't track down anything wrong. I suspect there might be something in core maybe?

[pinkisemils]

Reviewed 6 of 12 files at r1.
Reviewable status: 6 of 12 files reviewed, 1 unresolved discussion