Always use curlx_now() when calling Curl_pgrs_now(data). Tests with the
"manual" updates to now proved differ more then 100ms in parallel testing.
Add `curlx_nowp()` to set current time into a struct curltime.
Add `curlx_ptimediff_ms() and friends, passing pointers.
Update documentation.
Closes#19998
Use `data->progress.now` as the timestamp of proecssing a transfer.
Update it on significant events and refrain from calling `curlx_now()`
in many places.
The problem this addresses is
a) calling curlx_now() has costs, depending on platform. Calling it
every time results in 25% increase `./runtest` duration on macOS.
b) we used to pass a `struct curltime *` around to save on calls, but
when some method directly use `curx_now()` and some use the passed
pointer, the transfer experienes non-linear time. This results in
timeline checks to report events in the wrong order.
By keeping a timestamp in the easy handle and updating it there, no
longer invoking `curlx_now()` in the "lower" methods, the transfer
can observer a steady clock progression.
Add documentation in docs/internals/TIME-KEEPING.md
Reported-by: Viktor Szakats
Fixes#19935Closes#19961
