Trigger resource events on destination collection/records

[leplatrem]

This is major rewrite of the updater in order to trigger ResourceChanged events when the destination collection/records are updated (on signing).

The challenges are:

• Leverage the resource code that builds and sends events
• Trigger a ResourceChanged from a ResourceChanged may not be natural :)
• Rewrite the tests since they rely on mocks and are broken

I chose to go with invoque_subrequest() instead of the kind of awful code we currently have in the default bucket plugin.

I find the result convincing and promising (most events tests pass), but I need feedback :)

@almet @Natim f?

[Natim]

Using the public API instead of the storage one seems like a good idea but it means you will need to have write permission for a given, while the aim was to make sure only the kinto-signer could modify the collection.

Isn't it possible to trigger the ResourceChanged event instead?
If you want to keep this approch, we need a mecanism that let plugins bypass the permission check.

[leplatrem]

Ok, so that means it lacks a test where two different people modify the source collection, and the destination is updated regardless who created the destination first.