before:

• /var/run/memcached is a bad default for a socket path, since its
  parent directory must be writeable by memcached.
• Socket directory was not created by the module itself -> this was
  left as a burden to the user?
• Having a static uid with a dynamic user name is not very useful.

after:

• Replace services.memcached.socket by a boolean flag. This simplifies
  our code, since we do not have to check if the user specifies a
  path with a parent directory that should be owned by memcached
  (/run/memcached/memcached.sock -> /run/memcached).
• Remove fixed uid/gid allocation. The only file ever owned by the
  daemon is the socket that will be recreated on every start.
  Therefore user and group ids do not need to be static.
• only create the memcached user, if the user has not specified a
  different one. The major use case for changing option is to allow
  existing services (such as php-fpm) opening the local unix socket.
  If we would unconditionally create a user that option would be
  useless.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.