eth/downloader: also drain stateCh, receiptCh in eth/61 mode

State and receipt deliveries from a previous eth/62+ sync can hang if
the downloader has moved on to syncing with eth/61. Fix this by also
draining the eth/63 channels while waiting for eth/61 data.

A nicer solution would be to take care of the channels in a central
place, but that would involve a major rewrite.