In this commit, we replace the existing min/max duration system with a
single field that expresses the expected duration of a lease. Note that
we've left some of the exiting logic in place that select a duration
from a bid in an attempt to make this more of a "soft fork" which will
allow any existing orders to be cleared out as normal before we switch
over.
Once we add multiple durations (this commit assumes a single duration),
we'll need to modify the verification logic to be able to handle
multiple clearing prices for a given batch.
As a preparation to avoid package import cycles when introducing more
generic terms, we move the FeeSchedule interface and its
LinearFeeSchedule implementation to the terms package.
