Capped collections support for Mongo Appender

Mongo database now support capped collection. Capped collections are fixed-size collections that support high-throughput operations that insert, retrieve, and delete documents based on insertion order. Capped collections work in a way similar to circular buffers: once a collection fills its allocated space, it makes room for new documents by overwriting the oldest documents in the collection.

For more information see: http://docs.mongodb.org/manual/core/capped-collections/