DBCursor's OptionalFinalizer inner class is now static and only contains fields for a Mongo instance and a ServerCursor instance.
Doing this prevents it from holding references to the DBCursor state which it does not actually need, allowing the garbage collector
to reclaim that memory before the finalizer runs.

 JAVA-1519
Merge pull request #272 from jyemin/j1519

JAVA-1519: DBCursor's OptionalFinalizer inner class is now static ...