table: block: fix some racy condition in interator and some cleanup
table: block: fix some racy condition in interator and some cleanup

chai2010/goleveldb,hustbill/goleveldb,FactomProject/goleveldb,conseweb/goleveldb,rentongzhang/goleveldb,zzl0/goleveldb,trigrass2/goleveldb,syndtr/goleveldb,btcsuite/goleveldb,ngaut/goleveldb,no2key/goleveldb