refactor: store `readPreference` in operation options
refactor: store `readPreference` in operation options

This should go away in the future, but our codebase has many baked
in assumptions that the readPreference will always be preesnt in
passed in `options`. We also have a number of tests that directly
inspect these values, which should be corrected in the future.

mongodb/node-mongodb-native,mongodb/node-mongodb-native,mongodb/node-mongodb-native,mongodb/node-mongodb-native