miner, consensus/clique: avoid memory leak during block stasis (#23861)

This PR fixes a problem which arises on clique networks when there is a network stall. Previously, the worker packages were tracked, even if the sealing engine decided not to seal the block (due to clique rules about recent signing). These tracked-but-not-sealed blocks kept building up in memory. 
This PR changes the situation so the sealing engine instead returns an error, and the worker can thus un-track the package.

miner/worker.go

@@ -593,6 +593,9 @@ func (w *worker) taskLoop() {
 
 			if err := w.engine.Seal(w.chain, task.block, w.resultCh, stopCh); err != nil {
 				log.Warn("Block sealing failed", "err", err)
+				w.pendingMu.Lock()
+				delete(w.pendingTasks, sealHash)
+				w.pendingMu.Unlock()
 			}
 		case <-w.exitCh:
 			interrupt()