perform initial notify when adding agent to agency

That way when we add new agent with some initial value,
other agents get chance to immediately react and don't have to
wait for next value change, which may never come.

Signed-off-by: Jan Dvořák <mordae@anilinux.org>
make initial agent value mandatory

Signed-off-by: Jan Dvořák <mordae@anilinux.org>