nix-store -r: do substitutions in parallel

I.e. when multiple non-derivation arguments are passed to ‘nix-store
-r’ to be substituted, do them in parallel.

src/libstore/remote-store.hh

@@ -60,7 +60,7 @@ public:
 
     Paths importPaths(bool requireSignature, Source & source);
     
-    void buildDerivations(const PathSet & drvPaths);
+    void buildPaths(const PathSet & paths);
 
     void ensurePath(const Path & path);