localThresholdMS should be a global immutable value

Currently local threshold is set as an option for individual instances of read preference classes:

# PyMongo 3.0-dev, removed by this change.

from pymongo.read_preferences import Secondary

rp = Secondary(latency_threshold_ms=20000)

This ticket tracks making localThreshold global and immutable. That is, it can only be set through the URI option localThresholdMS, or passed to the constructor for MongoClient as a keyword argument:

client = pymongo.MongoClient('mongodb://localhost:27017/?localThresholdMS=20000')

client = pymongo.MongoClient(localThresholdMS=20000)

It can not be reset after client creation and is not settable per Database, Collection, or operation. mongos does not support per operation changes to localThreshold through the $readPreference directive, so the existing driver API can lead to dramatic behavior changes when migrating from a standalone mongod or replica set to a sharded cluster.