Make sure bundles are created in a single txn

[epicfaace]

codalab-worksheets/codalab/model/bundle_model.py

Lines 1235 to 1239 in 2969c01

	with self.engine.begin() as connection:
	result = connection.execute(cl_bundle.insert().values(bundle_value))
	self.do_multirow_insert(connection, cl_bundle_dependency, dependency_values)
	self.do_multirow_insert(connection, cl_bundle_metadata, metadata_values)
	bundle.id = result.lastrowid