Proxy Request-Id to stagecraft

As someone operating a distributed system
I would like to be able to trace a single request through everywhere
So that I have good transparency in how the system behaves
Log Request-Id for all incoming requests

See https://docs.python.org/2/library/logging.html#logging.Logger.debug
and https://github.com/exoscale/python-logstash-formatter/blob/440a7c22beb2e51b07b978ea09cd8f69b136ccfd/logstash_formatter/__init__.py
Merge pull request #357 from alphagov/feature/log-request-id

Log Request-Id