Improve the multithread support

[lu-zero]

Currently we have some of issue that are well known:

• rayon can be better
• compute_lookahead_data should happen in a separate thread

[lu-zero]

The upcoming rayon solves its problems neatly:

And we have compute_block_importances and compute_lookahead_data that could happily live in a separate thread and not stall the rest of the encoding.

[KyleSiefring]

I believe compute_block_importances could be split into multiple threads similar to tiling.

[shssoichiro]

Ah, compute_block_importances should be tiled anyway or the block importances will be inaccurate for blocks that move across tile boundaries.

Edit: This might be incorrect, because it looks like we do use tiles in compute_lookahead_motion_vectors which is one of the data points block importances are based on and the one that would impact this.

[lu-zero]

8cc2268 and 1e0a8ce resulted in ~21% speedup over Chimera 1080p with 8 tiles and >8 threads.