Design and implement new DataSources API

We need 2 distinct DataSources:

• DataStream with only an append operation
• DataStore with all 3 WMA operations (has to return a FH)

Each RuleUnit will have a mailbox. When a RuleUnit is executed for the first time all data in its DataSources are flushed into its mailbox (wrapped in Commands). Then, when the RuleUnit is quiescent, it remains subscribed to its DataSources so that all new operation performed on them get also automatically flushed into its mailbox.