src: introduce internal C++ SetImmediate() and remove async_hooks destroy timer

Introduce env->SetImmediate(fn) and remove the handle we use for async_hooks destroys.

After #17105, a similar thing could be done for HTTP/2.

(The one test change is due to the slightly different timing for async_hooks, since timer and check handle callbacks run at different times.)

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

src