Use virtual threads instead of a thread pool

[amanteaux]

Virtual threads are becoming a reality since Java 19, though currently they are only available through preview usage of the JVM: --enable-preview.

When virtual threads are fully part of the JVM, Wisp core should be rewritten to use virtual threads:

• It should actually consume less hardware resources
• The core code will be smaller and easier to maintain
• There will be no more need to configure the thread pool since virtual thread pooling is not recommended