Atomic counter support

Atomicity of increment/decrement operations
Now MagnetoDB has partially implemented UpdateItem request. It has support of PUT and DELETE actions on storage level which don't not depend on current type of item's attribute to be updated. We need to Implement smart UpdateIdem ADD action which will be able to perform atomic decrement and increment if Item's attribute for update operation is number. Take a look at DynamoDB API documentation for details:
http://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_UpdateItem.html

Also now MagnetoDB doesn't have REST API implementation of UpdateItem request, So it is also necessary to implement it.