fixes Bug 836753: added 'throttle_rate' to raw crash

this PR adds a new field, 'throttle_rate', to the raw crash.  When a crash is collected, it is passed through the throttler.  The throttler makes the decision if a crash is to be processed or not.  When compiling stats and comparing different versions, it is useful to know what throttle value was used for a given crash.  That rate is now saved in the raw_crash as the value of 'throttle_rate'