small important change to clustering functionality

Hello, I tried to change the ClusterCheckinInterval property for JobStoreTx , but I was not able to change it less then 7.5 seconds. If I set the property 

quartz.jobStore.clusterCheckinInterval to value 1000

and switch off one cluster node, another one goes into action only 7.5 seconds later. I found the problem in CalcFailedIfAfter method in JobStoreSupport class and fixed it.

Thanks.