Feature: Allow deletions on aggregate states

[amalcaraz]

Problem:

Using aggregates fits perfectly for having unique key/value sets of entities. We can create and update them, but without supporting deletions the only option that a user has is to set values to null or some arbitrary value that identifies this "deleted" status.

Doing this logical deletion brings to problems:

• The user need to parse the aggregation and filter this values on client side each time the it is fetched from the API
• The orphans keys consume unnecessary resources on the CCN

Opinion:

Would be nice to have a way to delete this keys instead marking them as null. But legacy behavior is to keep all keys even if the value is null, and in some cases a use could want to have a null value associated to a key without loosing that key

Proposal:

Either:

• Implement an improvement on aggregate logic to define an arbitrary value FORGET for example, to delete keys by merging a new message:

{ foo: 1, bar: 2 } + { foo: 'FORGET' } = { bar: 2}

• Create an additional endpoint for squashing null values on demand from an aggregation without interfering on the current behavior