Move cbuildbot_background to chromite.lib for easier reuse.

cbuildbot_background is already reused in many scripts outside of the
cbuildbot namespace, so it should be moved into lib/.

BUG=none
TEST=pylint, grep, all unit tests.

Change-Id: I57106c2787cad82c84b06fb970ce83b931a52896
Reviewed-on: https://gerrit.chromium.org/gerrit/39188
Commit-Ready: David James <davidjames@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
Tested-by: David James <davidjames@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 2f0137a..4985702 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -21,7 +21,6 @@
 import time
 
 from chromite.buildbot import builderstage as bs
-from chromite.buildbot import cbuildbot_background as background
 from chromite.buildbot import cbuildbot_config
 from chromite.buildbot import cbuildbot_stages as stages
 from chromite.buildbot import cbuildbot_results as results_lib
@@ -39,6 +38,7 @@
 from chromite.lib import git
 from chromite.lib import osutils
 from chromite.lib import patch as cros_patch
+from chromite.lib import parallel
 from chromite.lib import sudo
 
 
@@ -374,7 +374,7 @@
                            suite])
 
     steps = [self._GetStageInstance(*x, config=config).Run for x in stage_list]
-    background.RunParallelSteps(steps + [archive_stage.Run])
+    parallel.RunParallelSteps(steps + [archive_stage.Run])
 
   def RunStages(self):
     """Runs through build process."""
@@ -404,7 +404,7 @@
       # This process runs task(board) for each board added to the queue.
       queue = multiprocessing.Queue()
       task = self._RunBackgroundStagesForBoard
-      with background.BackgroundTaskRunner(queue, task):
+      with parallel.BackgroundTaskRunner(queue, task):
         for board in self.build_config['boards']:
           # Run BuildTarget in the foreground.
           archive_stage = self.archive_stages[board]