Init DeviceIdentifier to fixed value in CLI without setting cookies

[acoulton]

This avoids the risk of errors setting the cookie if the process has
already sent output to the console.

[acoulton]

@craig410 this isn't quite as clean as I'd like, obviously want it to detect the CLI automatically but that state needs to be injected into the class otherwise it prevents all the unit tests running the real code.

Which means I can only really pass it into the init method - if a logger calls ::get() before it initialises (e.g. due to an error early in bootstrap) it will get -unset- instead of the CLI value. But I think that's a tolerable edge case for keeping it a small change and testable?

[craig410]

Yeah I agree, theres not any other way to do it and overall it's neater than having to wrap every usage of it per application