Improve Relay command encoding scheme

[kevsmith]

Relay uses Go's encoding/gob to send invocation requests and receive invocation responses. It turns gob is one of the most verbose and slowest binary encodings currently available for Go.

A minimal command invocation request (executable set to "/bin/date", no Cog args, no Cog opts) weighs in at 171 bytes when gob encoded. This is surprisingly large.

[kevsmith]

Based on Alec Thomas' Go serialization benchmarks I've replaced gob with gogo/protobuf as this seems to be both the fastest codec and marshaling scheme available.

The same minimal command invocation serializes to only 40 bytes. This represents a 76.6% decrease in payload size.