Connections can be dropped by the net_node layer,
unbeknownst to cryptonote_protocol, which would then
not flush any spans scheduled to that connection,
which would cause it to be only downloaded again
once it becomes the next span (possibly after a small
delay if it had been requested less than 5 seconds
ago).
A block queue is now placed between block download and
block processing. Blocks are now requested only from one
peer (unless starved).
Includes a new sync_info coommand.
