miner: regenerate mining work every 3 seconds (#17413)

* miner: regenerate mining work every 3 seconds

* miner: polish

core/events.go

@@ -29,9 +29,6 @@ type PendingLogsEvent struct {
 	Logs []*types.Log
 }
 
-// PendingStateEvent is posted pre mining and notifies of pending state changes.
-type PendingStateEvent struct{}
-
 // NewMinedBlockEvent is posted when a block has been imported.
 type NewMinedBlockEvent struct{ Block *types.Block }