Add counter support in the mapper

Resolution:

We eventually settled on this API:

Initial description:
From this stackoverflow question.

It's possible to map an entity to a counter table, but currently this only works for read and delete operations. For updates, you need to use @Query in order to provide the custom increment/decrement clause:

We could provide something a bit more user-friendly with a new annotation:

(The current counter values in the entity are ignored, it's just used for the primary key – like @Delete)

The API needs a bit more thought:

• there can be multiple counter columns

• the increment can be different than 1, it can probably be bound as well

• do we need a separate annotation for decrementation?