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.
The ReservedValue method returns the worst case value the must be
available in the account to ensure the trader can pay for the order
including its fees.
