Push wopAddToStore old style stream adapters into smaller scopes
Push wopAddToStore old style stream adapters into smaller scopes

This doesn't fix the bug, but makes the code less difficult to read.
Also improve the comments, now that it is clear what part is needed in
each code path.

NixOS/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,rycee/nix