Refine `dhfind` to be a double hash proxy

[masih]

dhfind contains simulation mode and only answers double hashed queries. Once dhstore and dhfind are combined, the code should be refactored to offer a dh read proxy of sorts for general use: the proxy should:

• handle lookups gracefully and do the right thing depending on multihash code.
• remove simulation functionality and be a simple HTTP proxy.