Add caching, compressing, and CORS-allowing RPC proxy middleware

[0xB10C]

It would be nice to be able to load addrman tables directly from Bitcoin Core RPC. However, accessing Bitcoin Core RPC from a Browser ins't something we want to do. Better to have a proxy that handles credentials and only ever allows access to the getrawaddrman RPC.

The proxy should:

• proxy the JSON received from RPC to a requester handling e.g. the RPC credentials
• it might cache responses for a few seconds (e.g. up to 60 seconds)
• support multiple Bitcoin Core nodes (one proxy instance for n nodes)
• take care of compressing (gzip) the response if the requester supports that (getrawaddrman JSON output is highly compressible with a factor of about 10x)
• respond to requests with the Access-Control-Allow-Origin: * header set to allow cross-origin fetch requests

[0xB10C]

The caching part might not be needed in the proxy. This reduces implementation complexity. You only need a cache if you expose the proxy publicly, e.g. with nginx. At this point you can also use nginx to handle the caching.