Low-latency closure copy

This adds a new store operation 'addMultipleToStore' that reads a
number of NARs and ValidPathInfos from a Source, allowing any number
of store paths to be copied in a single call. This is much faster on
high-latency links when copying a lot of small files, like .drv
closures.

For example, on a connection with an 50 ms delay:

Before:

  $ nix copy --to 'unix:///tmp/proxy-socket?root=/tmp/dest-chroot' \
    /nix/store/90jjw94xiyg5drj70whm9yll6xjj0ca9-hello-2.10.drv \
    --derivation --no-check-sigs
  real    0m57.868s
  user    0m0.103s
  sys     0m0.056s

After:

  real    0m0.690s
  user    0m0.017s
  sys     0m0.011s

src/libstore/store-api.hh

@@ -440,6 +440,12 @@ public:
     virtual void addToStore(const ValidPathInfo & info, Source & narSource,
         RepairFlag repair = NoRepair, CheckSigsFlag checkSigs = CheckSigs) = 0;
 
+    /* Import multiple paths into the store. */
+    virtual void addMultipleToStore(
+        Source & source,
+        RepairFlag repair = NoRepair,
+        CheckSigsFlag checkSigs = CheckSigs);
+
     /* Copy the contents of a path to the store and register the
        validity the resulting path.  The resulting path is returned.
        The function object `filter' can be used to exclude files (see