Changed backend logics for block finding and payout

* Findblocks cronjob changes
 * Find & Store upstream share
  * Use last found `share_id` as starting ID or `0`
 * Find & Store upstream finder
  * Use last found `share_id` as starting ID or `0`
* Use stored information when running propotional payout
 * Fetch current checked blocks upstream share from block table
 * Fetch previous upstream share of previous block from block table
 * Calculated payouts in that range of IDs
* Updated `block.class.php` to store share_id in block and fetch highest `share_id`
Merge pull request #83 from TheSerapher/refactor-backend

Changed backend logics for block finding and payout