Add a function queryValidPaths()

queryValidPaths() combines multiple calls to isValidPath() in one.
This matters when using the Nix daemon because it reduces latency.
For instance, on "nix-env -qas \*" it reduces execution time from 5.7s
to 4.7s (which is indistinguishable from the non-daemon case).

src/libstore/store-api.hh

@@ -113,6 +113,9 @@ public:
     /* Check whether a path is valid. */ 
     virtual bool isValidPath(const Path & path) = 0;
 
+    /* Query which of the given paths is valid. */
+    virtual PathSet queryValidPaths(const PathSet & paths) = 0;
+
     /* Query the set of all valid paths. */
     virtual PathSet queryAllValidPaths() = 0;