fix: discovery AyncFilter deadlock on shutdown

[zzzckck]

Description

Hi, we found a occasionally node hang issue on BSC, I think Geth may also have the issue, so pick the fix patch here.
The fix on BSC repo: bnb-chain/bsc#3347

When the hang occurs, there are two routines stuck.

• routine 1: AsyncFilter(...)
  On node start, it will run part of the DiscoveryV4 protocol, which could take considerable time, here is its hang callstack:

goroutine 9711 [chan receive]:  // this routine was stuck on read channel: `<-f.slots`
github.com/ethereum/go-ethereum/p2p/enode.AsyncFilter.func1()
	github.com/ethereum/go-ethereum/p2p/enode/iter.go:206 +0x125
created by github.com/ethereum/go-ethereum/p2p/enode.AsyncFilter in goroutine 1
	github.com/ethereum/go-ethereum/p2p/enode/iter.go:192 +0x205

• Routine 2: Node Stop
  It is the main routine to shutdown the process, but it got stuck when it tries to shutdown the discovery components, as it tries to drain the channel of <-f.slots, but the extra 1 slot will never have chance to be resumed.

goroutine 11796 [chan receive]: 
github.com/ethereum/go-ethereum/p2p/enode.(*asyncFilterIter).Close.func1()
	github.com/ethereum/go-ethereum/p2p/enode/iter.go:248 +0x5c
sync.(*Once).doSlow(0xc032a97cb8?, 0xc032a97d18?)
	sync/once.go:78 +0xab
sync.(*Once).Do(...)
	sync/once.go:69
github.com/ethereum/go-ethereum/p2p/enode.(*asyncFilterIter).Close(0xc092ff8d00?)
	github.com/ethereum/go-ethereum/p2p/enode/iter.go:244 +0x36
github.com/ethereum/go-ethereum/p2p/enode.(*bufferIter).Close.func1()
	github.com/ethereum/go-ethereum/p2p/enode/iter.go:299 +0x24
sync.(*Once).doSlow(0x11a175f?, 0x2bfe63e?)
	sync/once.go:78 +0xab
sync.(*Once).Do(...)
	sync/once.go:69
github.com/ethereum/go-ethereum/p2p/enode.(*bufferIter).Close(0x30?)
	github.com/ethereum/go-ethereum/p2p/enode/iter.go:298 +0x36
github.com/ethereum/go-ethereum/p2p/enode.(*FairMix).Close(0xc0004bfea0)
	github.com/ethereum/go-ethereum/p2p/enode/iter.go:379 +0xb7
github.com/ethereum/go-ethereum/eth.(*Ethereum).Stop(0xc000997b00)
	github.com/ethereum/go-ethereum/eth/backend.go:960 +0x4a
github.com/ethereum/go-ethereum/node.(*Node).stopServices(0xc0001362a0, {0xc012e16330, 0x1, 0xc000111410?})
	github.com/ethereum/go-ethereum/node/node.go:333 +0xb3
github.com/ethereum/go-ethereum/node.(*Node).Close(0xc0001362a0)
	github.com/ethereum/go-ethereum/node/node.go:263 +0x167
created by github.com/ethereum/go-ethereum/cmd/utils.StartNode.func1.1 in goroutine 9729
	github.com/ethereum/go-ethereum/cmd/utils/cmd.go:101 +0x78

The rootcause of the hang is caused by the extra 1 slot, which was designed to make sure the routines in AsyncFilter(...) can be finished. This PR fixes it by making sure the extra 1 shot can always be resumed when node shutdown.