Fix message callback index.
Merge pull request #785 from brendandahl/message-idx

Fix message callback index.
Improve Windows performance immensely

Busy-waiting for async reads (assuming system has 2 cores) improves the
performance due to numerous acks in ADB's protocol. Before we were waiting
100ms before every ack artificially slowing down the system.

If we need better performance on systems with one core, we should change
this to Sleep(1) (but even 1ms sleeping slows down `adb push`
considerably).