Make Gridseed and Zeus drivers use overlapped I/O on Windows.

Necessary because by default simultaneous reads and writes from
different threads will block on Windows.
Use calculated hashrate until first result is returned.

Prevents cgminer from marking miners SICK initially when
difficulty is high. Mostly of concern for solo mining.
Make possible solo mining without specifying a target address.

The getblocktemplate protocol requires specifying a block reward
target address under certain circumstances. If no address is
provided, fall back to using getwork protocol instead of raising
an error.
Quell compiler warnings