Update Cowboy to 0.8.3 and Ranch to 0.8.0

For chunk size support and much faster connecting speed.
Use the configured chunk size when streaming
Don't use set_resp_body unless strictly required

Now the body is starting to be sent on the same packet as the
response instead of after waiting the TCP round-trip, which
means the client receives a little faster.

This will probably not have an impact on benchmarks except on
small to medium files.