fix bulk_write context manager.
don't update ancestors while bulk editing.

Conflicts:
	common/lib/xmodule/xmodule/modulestore/mongo/base.py
Merge pull request #4477 from edx/studio/minimal-fix-import-performance

Studio/minimal fix import performance
Merge remote-tracking branch 'origin/release' into rc/2014-07-16

Conflicts:
	common/lib/xmodule/xmodule/modulestore/mixed.py
	common/lib/xmodule/xmodule/modulestore/mongo/base.py