PYTHON-719 Read preference backward compatibility

This commit does a few things:

- Adds tag_sets back (deprecated)
- Adds secondary_acceptable_latency_ms back (deprecated)
- Makes acceptable latency a per read preference setting
- Cleans up read preference instance generation
- Adds latencyThresholdMS as an alias for secondaryAcceptableLatencyMS.
The name may change before 3.0 is released.