Make LocalBinaryCacheStore::narFromPath() run in constant memory

This reduces memory consumption of

  nix copy --from file://... --to ~/my-nix /nix/store/95cwv4q54dc6giaqv6q6p4r02ia2km35-blender-2.79

from 514 MiB to 18 MiB for an uncompressed binary cache, and from 192
MiB to 53 MiB for a bzipped binary cache. It may also be faster
because fetching can happen concurrently with decompression/writing.

Continuation of 48662d151b.

Issue https://github.com/NixOS/nix/issues/1681.

src/libutil/util.hh

@@ -98,6 +98,7 @@ unsigned char getFileType(const Path & path);
 /* Read the contents of a file into a string. */
 string readFile(int fd);
 string readFile(const Path & path, bool drain = false);
+void readFile(const Path & path, Sink & sink);
 
 /* Write a string to a file. */
 void writeFile(const Path & path, const string & s, mode_t mode = 0666);