PYTHON-526 secondaryAcceptableLatencyMS changes.

- Renamed to acceptableLatencyMS since it is also used
in choosing a new mongos during mongos HA failover.
- Moved to a global immutable setting. The expectation is
that the value would be determined by a network/system
admin. Changing it per operation doesn't make a lot of
sense.

Note - it's possible the name of this option may change again
before 3.0 is released.