Due to occasional bad network conditions, a node can miss a block.
When the next one is received and detected orphaned, we should at least try a request for the
previous block, assuming it's not already an orphan in itself and we're not syncing already.
No additinal tests were implemented, test like `simulate_full_sync` test this functionality because
sync starts later. It seems to be an issue with sync test coverage. As fix I'd suggest to start sync
as soon as we get a peer connection.
[https://github.com/mimblewimble/grin/issues/705]
Alexey Miroshkin
452996a421