Panic in startChainDownload

[cpacia]

There's a likely race condition in the startChainDownload() function that is occasionally resulting in close() being called on the blockQueue channel more than once causing a panic. For now the panic his handled by a recover() but some investigation is needed to figure out why it's reaching that point in the code more than once when it shouldn't. More broadly this part of the code could stand to use some refactoring so it isn't as racey.