gridseed: Switch from the Queued miner loop to the Scanhash miner loop
Improves performance on Raspberry Pi
Fixes inaccurate hashrate estimates
Fixes bug overwriting work inadvertently
Merge pull request #445 from nwoolls/feature/gridseed-scanhash-loop

gridseed: Switch from the Queued miner loop to the Scanhash miner loop