MEDIUM: sample: fill the struct sample with the session, proxy and stream pointers

MEDIUM: sample: fill the struct sample with the session, proxy and stream pointers

Some sample analyzer (sample-fetch or converters) needs to known the proxy,
session and stream attached to the sampel. The sample-fetches and the converters
function pointers cannot be called without these 3 pointers filled.

This patch permits to reduce the sample-fetch and the converters called
prototypes, and provides a new mean to add information for this type of
functions.