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Part of the work to unblock: https://github.com/ipfs/go-log/pull/65

fixes #174

We don't need to return information about the actual HTTP error. That's usually
not something the user cares about (and is usually implied from the command).

This way, we get an actual context canceled error when the context is canceled.

This allows _trying_ to execute a command on a daemon, failing, and falling back
on another executor.

This is a breaking change and will affect Filecoin. However, filecoin is
currently using this incorrectly anyways (breaking PreRun and PostRun).

We _need_ to do this before calling PreRun but it's also nice to do this before
bothering the server.

This was causing cancel errors to be printed twice.

Only use custom encoders for values.

fixes #65

Also, remove SetEncoder. Always derive the correct encoder from the request.

to func(Response, ResponseEmitter) error

Instead, handle it at the boundaries (in ServeHTTP and the CLI's Run function).
This ensures that we actually *wait* to finish writing everything before
canceling the context.

Also, fix a nil pointer error.

fixes #42

We *always* expect environments to be able to return a context so we might as
well enforce this with the type system.

- made Response.Request() return Request not *Request
- added ResponseEmitter.Tee()
- added a wrapper that wraps a Response and provides an oldcmds.Response