AU: Speed up full update payload generation by running multiple threads.

Full end-to-end payload generation drops from 140 to 40 seconds on my machine.

BUG=8747
TEST=unit tests, updated through dev server

Change-Id: Ib3934b38a55e043abfa49abb25b2931e529ff768

Review URL: http://codereview.chromium.org/4610001
diff --git a/SConstruct b/SConstruct
index bd2a5d8..fd98b20 100644
--- a/SConstruct
+++ b/SConstruct
@@ -249,6 +249,7 @@
                    filesystem_iterator.cc
                    file_writer.cc
                    flimflam_proxy.cc
+                   full_update_generator.cc
                    graph_utils.cc
                    gzip.cc
                    libcurl_http_fetcher.cc
@@ -290,6 +291,7 @@
                             filesystem_copier_action_unittest.cc
                             filesystem_iterator_unittest.cc
                             flimflam_proxy_unittest.cc
+                            full_update_generator_unittest.cc
                             graph_utils_unittest.cc
                             http_fetcher_unittest.cc
                             mock_http_fetcher.cc