BUG/MAJOR: vars: always retrieve the stream and session from the sample

BUG/MAJOR: vars: always retrieve the stream and session from the sample

This is the continuation of previous patch called "BUG/MAJOR: samples:
check smp->strm before using it".

It happens that variables may have a session-wide scope, and that their
session is retrieved by dereferencing the stream. But nothing prevents them
from being used from a streamless context such as tcp-request connection,
thus crashing the process. Example :

    tcp-request connection accept if { src,set-var(sess.foo) -m found }

In order to fix this, we have to always ensure that variable manipulation
only happens via the sample, which contains the correct owner and context,
and that we never use one from a different source. This results in quite a
large change since a lot of functions are inderctly involved in the call
chain, but the change is easy to follow.

This fix must be backported to 1.6, and requires the last two patches.