Fetch current block heihgt on node start.

[volovyks]

Description

Now we are saving the last processed block in storage. If the node is offline for a long time, it starts 100500 signature generation protocols at once, which greatly affects our success rate.
@ppca proposition is to start at latest block on chain - 200 (max yield resume time).

We can index all blocks again if we add async requests that can wait indefinitely.