eth/protocols/eth: prevent hanging dispatch

[holiman]

This PR attempts to fix a strange test-failure (timeout) observed on a windows-32 platform.
https://ci.appveyor.com/project/ethereum/go-ethereum/builds/51174391/job/d8ascanwwltrlqd5

A goroutine is stuck trying to deliver a response:

goroutine 9632 [select, 29 minutes]:
github.com/ethereum/go-ethereum/eth/protocols/eth.(*Peer).dispatchResponse(0x314f100, 0x3e5f6d0, 0x3acbb84)
	C:/projects/go-ethereum/eth/protocols/eth/dispatcher.go:172 +0x2a5
github.com/ethereum/go-ethereum/eth/protocols/eth.handleBlockHeaders({0x12abe68, 0x30021b8}, {0x12a815c, 0x40b41c0}, 0x314f100)
	C:/projects/go-ethereum/eth/protocols/eth/handlers.go:301 +0x173
github.com/ethereum/go-ethereum/eth/protocols/eth.handleMessage({0x12abe68, 0x30021b8}, 0x314f100)
	C:/projects/go-ethereum/eth/protocols/eth/handler.go:205 +0x4f6
github.com/ethereum/go-ethereum/eth/protocols/eth.Handle({0x12abe68, 0x30021b8}, 0x314f100)
	C:/projects/go-ethereum/eth/protocols/eth/handler.go:149 +0x33
github.com/ethereum/go-ethereum/eth.testSnapSyncDisabling.func1(0x314f100)
	C:/projects/go-ethereum/eth/sync_test.go:65 +0x33
github.com/ethereum/go-ethereum/eth.(*handler).runEthPeer(0x30021b8, 0x314f100, 0x427f648)
	C:/projects/go-ethereum/eth/handler.go:355 +0xe65
created by github.com/ethereum/go-ethereum/eth.testSnapSyncDisabling in goroutine 11
	C:/projects/go-ethereum/eth/sync_test.go:64 +0x54f
FAIL	github.com/ethereum/go-ethereum/eth	1800.138s

I do not know if this is the proper fix, but it seems like a reasonable one to me. Ptal @fjl

[rjl493456442]

SGTM

[rjl493456442]

Btw, the TestSnapSyncDisabling68 is super weird, I will push something to improve the test in your PR.

[rjl493456442]

Let's merge it once the CI is green

[fjl]

I kind of want to check it before merging, just didn't have time yet.