cull Session digest history

a randomly selected ten percent is culled from the history

prevents infinite growth for long running sessions of many messages,
while maintaining reasonable replay protection.
Merge pull request #3382 from minrk/culldigest

cull Session digest history

a randomly selected ten percent is culled from the history

prevents infinite growth for long running sessions of many messages, while maintaining reasonable replay protection.

Size of digest is Configurable.
related to #1131