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/worker-protocol.hh

@@ -41,6 +41,7 @@ typedef enum {
     wopImportPaths = 27,
     wopQueryDerivationOutputNames = 28,
     wopQuerySubstitutablePathInfos = 29,
+    wopQueryValidPaths = 30,
 } WorkerOp;