Develop a specification of API to retrieve logs

[Kaffeine]

Support for server-side logs retrieval has to be implemented on the service side and it seems that the API would be very same for locally-saved logs.

Reasons to implement logs API on the service side:

1. Local-side and Server-side logs will be available via a single API.
2. For some protocols client application needs to download user data to operate properly (Telegram)
3. Only ConnectionManager can efficiently store the logs with all needed metadata.
4. Only ConnectionManager can efficiently retrieve logs with all needed metadata.

Sub tasks: #2, #3

[gkiagia]

Ideally this needs to be accompanied with a high level library API to store offline logs on the CM side, for protocols that don't do this on the server side.

An additional thought is required though on how to retrieve lots of log messages, or how to efficiently search in them. One reason telepathy-logger was implemented as a library instead of a dbus api, was exactly that it is more efficient that way.

Btw, we should have a solid plan about this before 1.0, since I will need to think about how to deal with tp-logger, which is apparently merged in telepathy-glib in 'next' (!)