Fix sync wedge corner case:
It could happen if a connection went into standby mode, while
it was the one which had requested the next span, and that span
was still waiting for the data, and that peer is not on the
main chain. Other peers can then start asking for that data
again and again, but never get it as only that forked peer does.
And various other fixes
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.
