MEDIUM: vars: Be able to parse parent scopes for variables

MEDIUM: vars: Be able to parse parent scopes for variables

Add session/stream scopes related to the parent. To do so, "psess", "ptxn",
"preq" or "pres" must be used instead of tranditionnal scopes (without the
first "p"). the "proc" scope is not concerned by this change because it is
not linked to a stream. When such scopes are used, a specific flags is added
on the variable description during the variable parsing.

For now, theses scopes are parsed and the variable description is updated
accordingly. But at the end, any operation on the variable value fails.