Add multiple (Service) Backends abstraction

[daringer]

This adds the structure to allow multiple service backends. Essentially the client now has the possibility to choose a order-of-dispatch in its ClientContext. This is an elementary step towards transparent hardware crypto devices like the se050.

• Extend ClientId to be a proper struct and call it make it a ClientContext (as the name suggests)
• Fix several concurrency issues during directory traversal using the ClientContext instead of a quasi-global
• extend all necessary components and reply_to dispatching based on client-chosen service backends

[sosthene-nitrokey]

I've rebased it on top of 8e347abf99dba81fca11c5779a473d263a7b0565 in sosthene-nitrokey:client-context-rebased