AU: Speed up updates by using buffered writes.

Given that SSD writes are really slow, this improves performance significantly.
This reduces update time on AGZ from 375 to 100 seconds.

BUG=6901
TEST=unit tests, gmerged on device and measured speed

Change-Id: Idac7743f6eaa8f26878a2d1f6b9401513e2ca600

Review URL: http://codereview.chromium.org/3471006
diff --git a/SConstruct b/SConstruct
index a9bf9eb..70803e7 100644
--- a/SConstruct
+++ b/SConstruct
@@ -193,6 +193,7 @@
   env['LIBS'] += ['bz2', 'gcov']
 
 sources = Split("""action_processor.cc
+                   buffered_file_writer.cc
                    bzip.cc
                    bzip_extent_writer.cc
                    cycle_breaker.cc
@@ -232,6 +233,7 @@
 unittest_sources = Split("""action_unittest.cc
                             action_pipe_unittest.cc
                             action_processor_unittest.cc
+                            buffered_file_writer_unittest.cc
                             bzip_extent_writer_unittest.cc
                             cycle_breaker_unittest.cc
                             decompressing_file_writer_unittest.cc