Use uWSGI instead of HTTP for proxy<->backend communication

[krzysdz]

TODO:

• check if remote address is passed properly
  • it isn't, I guess we'll have to keep using HTTP for now
• rebase after Update dependencies and remove OCSP stapling #53 is merged
• consider integrating certificate renewal using the native ACME protocol integration
  • NOT NOW
  • we're switching to nginx running on host, but the distro does not package the ACME module and I don't plan on building it on host (it's easier to do as a Docker image, because then I can easily guarantee that the module will always be built when nginx version changes)
• Remove proxy container and prepare configuration for host proxy #56