eth: fix hang in waitSnapExtension

[kaber2]

This patch fixes peers hanging in state 'eth: "handshake", snap: "handshake"' forever. On my nodes, I had a huge amount of these peers, which might indicate another issue, but this one clearly appears to be a bug.

Since this is my first Github PR, I'm not sure if this description will actually be the commit message as it pre-populated the PR text with my commit message, which is:

When a peer announcing snap support connects without actually starting
the snap protocol, waitSnapExtension() will wait forever.

Add a timer to stop waiting after 5s.

[kaber2]

Small addendum: it turns out the root cause for the large amount of occurences on my nodes were caused by local changes.

The fix itself still seems valid though.

[fjl]

I have discussed this with @karalabe, and we reached the following conclusion: The timeout is not necessary here because we are already checking whether "eth" is enabled earlier in waitSnapExtension:

go-ethereum/eth/peerset.go

Lines 86 to 88 in eb8ee67

	if !peer.RunningCap(eth.ProtocolName, eth.ProtocolVersions) {
	return errSnapWithoutEth
	}

If the eth is enabled, its handler will be launched, and so there is no need to wait with a timeout, it's more correct to rely on it sending on the channel.