Refacto conversation and message cache

[Swiip]

EDIT: final situation

Motivations: caches implementations for message and conversation was a bit messy. But more important, lots of event updates was missed.

• MessageProvider
  • Fix labels update management
• ConversationPriovider
  • Creation of this new provider to remove the hacky usage of the message cache for conversations
  • Add event manager subscription with support of labels changes
• useConversation
  • remove the use of useCachedModelResult for a manual management of loading request and cache updates.
• useMessage
  • restore immediate addition of a message to the cache unless event updates will be missed
• Add some tests
• Better type organization with event manager and cache types.

Remains a question about cache duplication in useElements and useMessage/useConversation. It will be addressed in another PR (if addressed)

[Swiip]

It seems to work at this point. Two things I want to precise:

I replaced the MessageProvider and replaced by a use***Cache which create a sort of sub-cache inside the main one. It seems simpler to me instead of stacking contexts. Update management for the cache is located next to it.

The useMessageCache and the useConversationCache are containing the same data as the localCache of useElements. It should be possible to link data from useElements to other cache to remove duplication. But it could be complicated for a "not so important" memory gain.

[Swiip]

After different attempts, I fallback on cache inside independants contexts. It's the best solution I found to manage futur eventual unloading.

That and new event management and some new tests. I think it's ok to merge now.

The question of cache duplication can be handled later.