start test mongod instance with --smallfiles

so mongo doesn't demand 3 GiB free in /tmp
clean up logging lines, removing or switching to debug level

asyncmongo throws exceptions and passes errors to callbacks, so the error
information is not lost, but the client application should be able to
control what is logged and how.

Some logging lines were already commented out, most of those were removed.
fix some pyflakes complaints about unused "e"