eth: fix transaction announce/broadcast goroutine leak

[karalabe]

The fail channels in the transaction announcement and broadcast loops were blocking, which meant that any announcement started must be allowed to finish before exiting the broadcaster. This was not the case as a peer drop concurrently closed the peer's termination channel and aborted the send. Depending on the ordering of the event, the outer broadcaster method might have reacted to the p.term closure faster, never reading the failure from fail.

One way to solve it would be to track any pending requests and only return from the outer method when the goroutine requests finish. This PR takes a simpler approach by converting the fail from a blocking channel to a notification one. This way if the sending goroutine is left dangling, it will just write into a noop channel.

[holiman]

lgtm