copyPaths(): Use queryValidPaths() to reduce SSH latency

src/build-remote/build-remote.cc

src/libstore/legacy-ssh-store.cc

src/libstore/local-store.cc

src/libstore/local-store.hh

src/libstore/remote-store.cc

src/libstore/remote-store.hh

src/libstore/store-api.cc

src/libstore/store-api.hh

src/nix-copy-closure/nix-copy-closure.cc

src/nix/copy.cc