Previously, Watch did not validate the "fullDocument" option and neglected to pass "readConcern" and "readPreference" to Aggregate. This refactors the class so that Watch only processes its own options.

Additionally, Aggregate is now first created from Watch::__construct() instead of Watch::execute(), which allows its options to be validated earlier (akin to FindOne creating Find in its constructor). Aggregate is only recreated if a new resume token is provided during a resume.