LocalStore::addToStoreFromDump copy in chunks

Rather than copying byte-by-byte, we let the coroutine know how much
data we would like it to send back to us.

src/libutil/serialise.hh

@@ -260,11 +260,20 @@ struct LambdaSource : Source
 /* Convert a function that feeds data into a Sink into a Source. The
    Source executes the function as a coroutine. */
 std::unique_ptr<Source> sinkToSource(
-    std::function<void(Sink &)> fun,
+    std::function<void(Sink &, size_t &)> fun,
     std::function<void()> eof = []() {
         throw EndOfFile("coroutine has finished");
     });
 
+static inline std::unique_ptr<Source> sinkToSource(
+    std::function<void(Sink &)> fun,
+    std::function<void()> eof = []() {
+        throw EndOfFile("coroutine has finished");
+    })
+{
+	return sinkToSource([fun](Sink & s, size_t & _) { fun(s); }, eof);
+}
+
 
 void writePadding(size_t len, Sink & sink);
 void writeString(const unsigned char * buf, size_t len, Sink & sink);