Implement group messages logic

[adambabik]

These changes implement logic of validating and handling new updates to the group.

Each update is verified, the events are validated and recomputed, and the chat is updated.

This PR contains one backward incompatible change. The signature of Messenger.Send changes to:

Send(ctx context.Context, chatID string, data []byte) ([][]byte, error)

because now Send can send multiple messages if a group chat has more than 1 member.

Limitations

• leaving group will also remove 1-1 chat, don't use :)
• no handling of members removal
• no managing group admins