Set a high priority for the read_thread on Mac.

On long consecutive reads we often see packet loss on the mac platform. We originally thought we were overrunning the queue of 30 packets hidapi uses, but with some debugging we realized we were losing the packets before then. After much experimentation, raising the priority of the read_thread() thread was the only way we could find to reliably get the data in time.

Signed-off-by: Phil Dibowitz <phil@ipom.com>