Block report times from datanodes could converge to same time.

Datanode block reports take quite a bit of memory to process at the namenode. After the inital report, DNs pick a random time to spread this load across at the NN. This normally works fine.

Block reports are sent inside "offerService()" thread in DN. If for some reason this thread was stuck for long time (comparable to block report interval), and same thing happens on many DNs, all of them get back to the loop at the same time and start sending block report then and every hour at the same time.

RPC server and clients in 0.18 can handle this situation fine. But since this is a memory intensive RPC it lead to large GC delays at the NN. We don't know yet why offerService therads seemed to be stuck, but DN should re-randomize it block report time in such cases.