db-parser: added .classifier.context-id for values that were correllated

This patch automatically sets ${.classifier.context-id} in messages that matched
a rule that contained a context-id attribute.

Reported-By: Fabien Wernli <bugzilla.balabit@faxm0dem.org>
Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>