Changes to queueing strategy for BitBurner boards

Because the BitBurner boards can accept partial work, sometimes cgminer ends up waiting (for avalon_buffer_full() to return false) while holding the qlock mutex. This can starve other threads which are waiting on qlock. This has two symptoms:
- stats aren't able to update, so the hashmeter remains stuck at 0.000,
- the queue isn't able to fill up, so miners are idled unnecessarily.

The proposed changes should encourage other threads to acquire qlock. AVALON_ARRAY_SIZE has also been increased (for BitBurner boards only) to stop "no matching work" errors from occurring.