Put in place content length checks to make problems more visible

[nickwilliams-eventbrite]

Issues with chunked message reassembly in the Redis transport, such as #240, would be easier to diagnose if we included chunk-length and content-length headers and performed sanity checks on received chunks and re-assembled messages with those headers. However, such a change cannot be included in the fix for #240, because it will require a new minor version and a new Redis protocol version (4).