Fix race condition in multithreaded miner

[vkoskiv]

If two threads find a block at the exact time (At a low difficulty), the client crashes. Should be a fairly simple fix.

[johndpope]

my attempt to fix.
#7

for some unknown reason -
if this is set to 00
while (!candidate.blockHash.binaryString.hasPrefix("00")) {
and usleep(20) is set here -
it silently bails out . not sure why.

sleep(1) or usleep(20000) - seems to work fine.