Feature Request Halt Mining

[BKW1882]

Feature suggestion: "halt": stops mining and waits for new "mine" command. Clears current block and hash information and wait for new challenge # to prevent wasted CPU/GPU cycles on resume.
Use case: solo miners who also use machine for CPU intensive tasks. Can be handled somewhat manually with setting affinity, but some tasks require stopping of mining to execute in timely fashion. Alternative: add affinity control to miner.