BROOKLYN-102: Fix MongoDB rebind, adding AttributeSensor.isTransient

Question for reviewer: is `isTransient` the best name for the field (+ method in the builder)? I'd normally have used `transient`, but that's a reserved word. And I didn't want to use the opposite (`persistable`) because folk are familiar with the concept of `transient` fields which is the closest analogy to this.
is it worth adding if it's null to return `REQUIRED`, to prevent old persisted items from returning `null` here?

or have an explicit `@Nullable`