This commit changes when the sweep pkscript
is assigned in the construction of the justice
transaction. Currently, the sweep pkscript is
assigned when the task is bound to a session.
However, we will moving to an assignment where
a unique sweep pkscript is used per channel to
prevent address inflation. Hence, this commit
makes the sweep pkscript a state dependent
variable, since it can be known at the time the
channel id is assigned.
