OGM-342 - Making write concern configurable for MongoDB

* Enhanced tuple operations in `GridDialect` to pass in a context providing access to the entity's options
* Making write concern configurable on global, entity and property level (the latter for associations stored in separate documents) via API and annotations
* Allowing to pass in a custom `WriteConcern` implementation in case the pre-defined options don't suffice

Any feedback is welcome :)