Changing hash- and share rate calculation SQL

* Do not include shares_archive table
 * Main reason: This table is optional
 * Secondary reason: Speeds up the query from 1.3s to 0.005s
* Drawback: Once a block is found it takes 10 minutes for the stats to
  be accuracte again

This could potentially be reverted but since shares_archive is optional
and the speed increase is rather significant I'd like to keep it this
way.