In this commit, we add a new key within the order bucket that stores
just the node tier for a given order. The current code only _expects_ a
node tier to exist for Bid orders, however things have been designed
such that, we can easily add tier options for Ask orders as well.
Along the way we modify the `orderCallback` to include the node tier
information as its read from a distinct bucket. In the future, we'll
likely want to make this into a struct so the function signature doesn't
ever expand.
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.
