MDL-63466 core_message: Add conversation support to get_messages

MDL-63466 core_message: Add conversation support to get_messages

- The get_conversation_messages has been added to the API and the WS
with the conversation identifier (convid) instead of the userto,
to let get conversation messages and the members involved.
- The cache has been also reviewed, to use the convid instead of the
[userfrom, userto] keys.
- The get_most_recent_conversation_messages has been added to the API
to update the cache when needed.