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/worker-protocol.hh

@@ -9,7 +9,7 @@ namespace nix {
 #define WORKER_MAGIC_1 0x6e697863
 #define WORKER_MAGIC_2 0x6478696f
 
-#define PROTOCOL_VERSION (1 << 8 | 31)
+#define PROTOCOL_VERSION (1 << 8 | 32)
 #define GET_PROTOCOL_MAJOR(x) ((x) & 0xff00)
 #define GET_PROTOCOL_MINOR(x) ((x) & 0x00ff)
 
@@ -55,7 +55,7 @@ typedef enum {
     wopQueryDerivationOutputMap = 41,
     wopRegisterDrvOutput = 42,
     wopQueryRealisation = 43,
-    wopImportPaths2 = 44, // hack
+    wopAddMultipleToStore = 44,
 } WorkerOp;