Test performance on the latest JDK-15-loom with use_fibers="true"/"false".

This requires a few code changes:

• There's no need to pool fibers, see [1] for details
• If fibers are used, thread_pool and internal_pool in TP could be assigned the result of Executors.newVirtualThreadExecutor()
• In a future version, we could simply remove the 2 thread pools

[1] https://cr.openjdk.java.net/~rpressler/loom/loom/sol1_part1.html