BUG/MEDIUM: vars: do not freeze the connection when the expression cannot be fetched

BUG/MEDIUM: vars: do not freeze the connection when the expression cannot be fetched

Commit 4834bc7 ("MEDIUM: vars: adds support of variables") brought a bug.
Setting a variable from an expression that doesn't resolve infinitely
blocks the processing.

The internal actions API must be changed to let the caller pass the various
flags regarding the state of the analysis (SMP_OPT_FINAL).

For now we only fix the issue by making the action_store() function always
return 1 to prevent any blocking.

No backport is needed.