Implement type-filtered backlog fetching

Add new backlog fetching methods that only return messages of the
specified types.  This allows clients to only fetch the types of
messages that they need, e.g. if join/part/quit is hidden.

This is not yet used in Quassel itself, but third-party clients
including QuasselDroid use this for better performance on limited
bandwidth networks.

Signed-off-by: Janne Koschinski <janne@kuschku.de>
Closes GH-339.