Refactoring Commands

Now that we have the `commands` package set up, it's time to move everything over to it. This PR is currently not ready for merge, just setting it up for CR.

The basic scope of this PR is as follows:
* Set up transports based on `commands` package (HTTP RPC server/client, CLI command handling)
* Refactor existing commands for the new API (won't be a 1:1 mapping, commands will change)
* Make changes to `commands` package when necessary
That's not true, in both cases the content type accurately describes the content. The content just happens to be 0 bytes long (but still a valid octet-stream or text file). However, some content types would be incorrect, such as "application/xml", where an empty string is not valid XML.