Investigate flakey test_rollback failures

[ShaneHarvey]

test_rollback sometimes fails with the below error. This could be a race condition in the test or a bug in rollback handling.

FAIL: test_rollback (tests.test_elastic.TestElastic)
Test behavior during a MongoDB rollback.

Traceback (most recent call last):
  File "/home/travis/build/mongodb-labs/elastic2-doc-manager/tests/test_elastic2.py", line 240, in test_rollback
    assert_soon(lambda: self._count() == 3)
  File "/home/travis/build/mongodb-labs/elastic2-doc-manager/.eggs/mongo_connector-2.4.1-py2.7.egg/mongo_connector/test_utils.py", line 260, in assert_soon
    raise AssertionError(message or "")
AssertionError