Add rolling ingestions stats

[brson]

We still have cases where block imports fall behind the chain and never catch up. Add some stats to show us more about what is happening.

During import keep a rolling calculation of

• imported blocks / second
• blocks / second being generated according to the timestamps on the imported blocks
• an estimate of time until import is complete based on number of blocks left
• an estimate of new blocks generated on-chain since import began
• an estimate of time to import all the new blocks since import began

Write these to disk and put them on the "log" page.