High-contention scenarios degrade performance with sagas

[helenktsai]

Issue transferred from internal repo

Sagas tend to experience severe performance degradation as the amount of contention increases. One common example of this is the "scatter-gather" pattern described in Jimmy Bogard's blog post: http://lostechies.com/jimmybogard/2014/02/27/reducing-nservicebus-saga-load/

Although improvements have been made by allowing users to make use of pessimistic locking for these scenarios, other improvements might still be possible.