lib: make `curlx_wait_ms()` and use it

lib: make `curlx_wait_ms()` and use it

Move function to curlx/, change all callers.

Also:
- src: replace local implementation.
- tests/client: replace local ad-hoc sleep code.
- tests/libtest: replace local `wait_ms()` implementation.
- lib1531: replace local ad-hoc sleep code.
- tests/server: replace local, simplified copy.
- tests/server: formatting, drop some unused headers.

Closes #17641