Fix test_aggregate for latest mongos.

mongos 2.6 has new $gleStats field in aggregation result.
Test that write concern of {wtimeout: 1000} is same as {w: 1, wtimeout: 1000}.
Use maxWriteBatchSize for write command batch splitting, PYTHON-642.
add failure case to test_auth.TestSASL
Add Luke Lovett to contributors.