MAJOR: sample: pass a pointer to the session to each sample fetch function

Many such function need a session, and till now they used to dereference
the stream. Once we remove the stream from the embryonic session, this
will not be possible anymore.

So as of now, sample fetch functions will be called with this :

   - sess = NULL,  strm = NULL                     : never
   - sess = valid, strm = NULL                     : tcp-req connection
   - sess = valid, strm = valid, strm->txn = NULL  : tcp-req content
   - sess = valid, strm = valid, strm->txn = valid : http-req / http-res
20 files changed