Add %byte{value} logformat code for logging or sending any byte (#1588)

Add %byte{value} logformat code for logging or sending any byte (#1588)

This feature is needed by at least the statsd tool receiving TCP log
info: https://github.com/statsd/statsd/blob/7c07eec/docs/server.md

No support for zero byte values yet because existing Format::assemble()
code does not support that out of the box, and there is no known need
for such support. It can be added later (without backward compatibility
problems) if needed.