profiling: use "stat" profiler to profile individual request The ls profiler no longer works for that. As the lsprof profiler is not default and not great is general, lets side step the issue for now.

profiling: improve 3.12 error message for calling lsprof twice Python 3.12 prevent lsprof to be enabled if it is already enabled. This break the use of lsprof in `hg serve` as both the initial `serve` command and the request serving want to profile. The "stat" profiler (the default) does not have this problem, so we focus on improving the error message for now.