Make entity controllers as persistent as possible

[DavidLeoni]

Currently ODR controllers for etypes, concepts, and in particular entities are supposed to be mutable and deletable, but this brings several design issues regarding traceability and persistence. Immutability and infinite persistence would solve many of them, and some work for it is on hold in TraceProv library (see TraceData and TraceDb)

Adopting TreceProv library would be of course a large change - for the time being especially for EntityController we can settle for this:

entity creation

For entity creation we have two different urls, one temporary for the new and one provided from the server when the entity is created, so we can right now easily create a copy of the controller.

entity read

This is more problematic, change of the controller can happen during entity cache refresh because the entity url is the same. So i.e. export reports might show latest entity information while actually they should show info about the entity at the time of the export.

entity update

This is even more problematic, but luckily we don't support it now.

entity deletion

Upon creation, controllers marked as new are currently deleted. Mmm not good! Better to keep them and start enforcing infinite persistence ( although this will clutter memory in the long run as ekb stays full in-ram!! See #210).