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

SConstruct

Index: SConstruct
diff --git a/SConstruct b/SConstruct
index bd2a5d8d45f3a3466973eed6ff2abda2dad9350a..fd98b2007d9a1ef04cb8918dd36b54e212172893 100644
--- a/SConstruct
+++ b/SConstruct
@@ -249,6 +249,7 @@ sources = Split("""action_processor.cc
                    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 @@ unittest_sources = Split("""action_unittest.cc
                             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