LocalStore: Allow the physical and logical store directories to differ

This is primarily to subsume the functionality of the
copy-from-other-stores substituter. For example, in the NixOS
installer, we can now do (assuming we're in the target chroot, and the
Nix store of the installation CD is bind-mounted on /tmp/nix):

  $ nix-build ... --option substituters 'local?state=/tmp/nix/var&real=/tmp/nix/store'

However, unlike copy-from-other-stores, this also allows write access
to such a store. One application might be fetching substitutes for
/nix/store in a situation where the user doesn't have sufficient
privileges to create /nix, e.g.:

  $ NIX_REMOTE="local?state=/home/alice/nix/var&real=/home/alice/nix/store" nix-build ...

src/libstore/local-store.hh

@@ -73,10 +73,12 @@ private:
 
     Sync<State, std::recursive_mutex> _state;
 
+    const Path realStoreDir;
     const Path dbDir;
     const Path linksDir;
     const Path reservedPath;
     const Path schemaPath;
+    const Path trashDir;
 
     bool requireSigs;