log truncate callback for storage trait

[Licenser]

Is your feature request related to a problem? Please describe.
This is related to #314 - the raft-rs callbacks do not include logic to truncate logs that the Leader knows to be applied to all followers. In combination with #320 this forces full snapshot replication to be the only viable form of catching up leaders.

Describe the solution you'd like
The leader keeps track of the last 'good' log on all followers, it seems sensible for the leader to truncate logs that are know to be replicated and applied to all Followers. A callback in the Storage trait seems to be useful.

Describe alternatives you've considered
Exposing all the needed information on the leader to introspect follower indexes and commits to manually implement truncation.