De-obfuscate the implementation of remove

It ended up being error-prone because of the ifdef.
Generate a X-Request-Id header for each request
This is needed to track the REST API requests end-to-end
and match notifications to API requests.
Make the request id header accessible on EnginioReply
to be able to use it in EnginioModel.
Do not expose the key generation function on EnginioBackendConnection
since it is not useful anywhere else but the WebSocket handshake.
Merge pull request #197 from bbandix/requestid

X-Request-Id