MINOR: sample: make RQ/RS stats available everywhere

MINOR: sample: make RQ/RS stats available everywhere

Sample fetch functions working on the request/response stats were marked
as being only compatible with the log phase. This is a mistake because
by definitions, stats can be consulted anywhere from the moment they
start to appear. It's only that they are valid as far as the logs. At the
moment, no sample fetch function depends on RQFIN, and only res.timer.data
depends on RSFIN. But this will be needed to relax certain sample fetch
functions (and will need to be backported along with a few other patches).