les: wait for all task goroutines before dropping the peer (#20010)

* les: wait all task routines before drop the peer

* les: address comments

* les: fix issue

les/test_helper.go

@@ -280,6 +280,8 @@ func newTestServerHandler(blocks int, indexers []*core.ChainIndexer, db ethdb.Da
 	}
 	server.costTracker, server.freeCapacity = newCostTracker(db, server.config)
 	server.costTracker.testCostList = testCostList(0) // Disable flow control mechanism.
+	server.clientPool = newClientPool(db, 1, 10000, clock, nil)
+	server.clientPool.setLimits(10000, 10000) // Assign enough capacity for clientpool
 	server.handler = newServerHandler(server, simulation.Blockchain(), db, txpool, func() bool { return true })
 	if server.oracle != nil {
 		server.oracle.start(simulation)