lib: keep timestamp in easy handle

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 #19935
Closes #19961

tests/data/test2500

@@ -72,7 +72,7 @@ via: 1.1 nghttpx
 s/^server: nghttpx.*\r?\n//
 </stripfile>
 <limits>
-Allocations: 150
+Allocations: 155
 Maximum allocated: 1800000
 </limits>
 </verify>