Need MongoDB Client to support ReadPreference at the Operation level.

[wdroidn7]

MongoDB supports the WriteConcern and ReadPreference at the DB, Collection, and Operation level.
The setting at the operation level overwrites the setting at the Collection level which overwrites the setting at the DB level.

The current Vertx MongoDB Client supports the WriteConcern at the DB, Collection and Operation level.

For Write operation, one can use the *WithOption() methods to specify the writeConcern. This is the operation level of WriteConcern which overwrites the collection and DB setting of the MongoClient.

For Query operation, one is limited to the readpreference setting when the MongoClient is created. To use different ReadPreference, one needs to create another MongoClient with different readPreference.