Support metadata as a specific case of data

Metadata is now just a specific case of data that is copied when it becomes unavailable and that is freed when a new value becomes available

• plugins now receive events for metadata
• no need for double data exposure in the example anymore
• keeping data for asynchronous processing should be quite similar to metadata handling (ref counting required)
• updated TODO

In order to update the plugins, I had to base this on branch feature-FTI_transaction, so please consider this after !20 (merged) only