MEDIUM: vars: Be able to retrieve variable of the parent stream, if any

MEDIUM: vars: Be able to retrieve variable of the parent stream, if any

It is now possible to retrieved the value of a variable using the parent
stream or the parent session instead of the current one. It remains
forbidden to set or unset this value. The sample fetch used to store the
result is a local copy. So it may be safely altered by a converter without
changing the value of the original variable.

Note that for now, the parent of a stream is never set. So this part is not
really used. This will change with the SPOE.