swarm/api: Multihash handler for resource updates in swarm api

[nolash]

The Mutable Resources backend treats multihash has a special data case, when the datalength is set to 0 (it has a special UpdateMultihash method that does it automatically).

However, the api has no way of specifying that an update is a multihash. Thus, upon retrieval, the http server cannot distinguish this also.

In cases where the content is a swarm multihash, it should automatically retrieve the resource the hash points to and serve that to the client instead.

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Currently, a raw resource update is made by POSTing data to bzz-resource://somename.eth.

One suggestion could be to have bzz-resource://somename.eth for multihashes, and bzz-resource://somename.eth/raw for others.

GET bzz-resource:// methods should also indicate if the response is raw or multihash, maybe using Content-Type header.

[cobordism]

How does this relate to: #186