There was a case where we had requested a block through the sync
mechanism and also received it through the normal inventory mechanism
where we would leave the peer in a sync state, but never ask them
for more sync blocks.
This commit fixes the bug that put us into that stuck state, and also
adds code to disconnect peers if we ever manage to get into that stalled
state.
one second (down from one full block interval), to reduce the chance of us getting
blocks out of order.
Remove the variable we use to keep track of the last block number a peer has;
This was an optimization in BitShares 0.x where looking up the number was expensive,
and maintaining it is error-prone.
