get-s3-ci-results inefficiently determines what needs to be scanned

Recently, get-s3-ci-results began failing with:
2016-07-17 23:54:15,659 ERROR [jujureports.jobs] BSON document too large (16782472 bytes) - the connected server supports BSON document sizes up to 16777216 bytes.

I have worked around this by breaking up a single request into multiple requests.
https://code.launchpad.net/~abentley/juju-reports/fix-list-no-match/+merge/300362

However, my workaround is just avoiding the size limitation, not fixing the inefficiency.

We have considered at least two alternatives that would be more efficient:
1. Only consider recent logs, e.g. those in the past 30 days
2. Only consider logs that sync_file considers new. This would be the most efficient, but is not an idempotent operation; sync_file will only ever return a given key once, so robustness can be an issue.

Full traceback:
2016-07-17 23:54:15,659 ERROR [jujureports.jobs] BSON document too large (16782472 bytes) - the connected server supports BSON document sizes up to 16777216 bytes.
Traceback (most recent call last):
  File "/home/ubuntu/juju-reports-repo/juju-reports-332-VxIJlv/local/lib/python2.7/site-packages/jujureports-0.0-py2.7.egg/jujureports/jobs/utils.py", line 208, in log_exceptions
    yield
  File "/home/ubuntu/juju-reports-repo/juju-reports-332-VxIJlv/local/lib/python2.7/site-packages/jujureports-0.0-py2.7.egg/jujureports/jobs/results.py", line 181, in main
    analyse_outcomes(ini, db, artifacts)
  File "/home/ubuntu/juju-reports-repo/juju-reports-332-VxIJlv/local/lib/python2.7/site-packages/jujureports-0.0-py2.7.egg/jujureports/jobs/results.py", line 144, in analyse_outcomes
    for attempt_key in list_no_match(db, attempts_for_logs(artifacts, 2592)):
  File "/home/ubuntu/juju-reports-repo/juju-reports-332-VxIJlv/local/lib/python2.7/site-packages/jujureports-0.0-py2.7.egg/jujureports/models.py", line 300, in list_no_match
    db.outcome_matches.find({'_id': {'$in': id_list}}, fields=[])
  File "/home/ubuntu/juju-reports-repo/juju-reports-332-VxIJlv/local/lib/python2.7/site-packages/jujureports-0.0-py2.7.egg/jujureports/models.py", line 299, in <setcomp>
    AttemptKey(**a['_id']) for a in
  File "/home/ubuntu/juju-reports-repo/juju-reports-332-VxIJlv/local/lib/python2.7/site-packages/pymongo/cursor.py", line 904, in next
    if len(self.__data) or self._refresh():
  File "/home/ubuntu/juju-reports-repo/juju-reports-332-VxIJlv/local/lib/python2.7/site-packages/pymongo/cursor.py", line 848, in _refresh
    self.__uuid_subtype))
  File "/home/ubuntu/juju-reports-repo/juju-reports-332-VxIJlv/local/lib/python2.7/site-packages/pymongo/cursor.py", line 782, in __send_message
    res = client._send_message_with_response(message, **kwargs)
  File "/home/ubuntu/juju-reports-repo/juju-reports-332-VxIJlv/local/lib/python2.7/site-packages/pymongo/mongo_client.py", line 1042, in _send_message_with_response
    response = self.__send_and_receive(message, sock_info)
  File "/home/ubuntu/juju-reports-repo/juju-reports-332-VxIJlv/local/lib/python2.7/site-packages/pymongo/mongo_client.py", line 1017, in __send_and_receive
    (request_id, data) = self.__check_bson_size(message)
  File "/home/ubuntu/juju-reports-repo/juju-reports-332-VxIJlv/local/lib/python2.7/site-packages/pymongo/mongo_client.py", line 929, in __check_bson_size
    (max_doc_size, self.__max_bson_size))
InvalidDocument: BSON document too large (16782472 bytes) - the connected server supports BSON document sizes up to 16777216 bytes.