Enhance order-submission to handle ask orders to be offered across multiple durations

**Is your feature request related to a problem? Please describe.**
Pool will soon be upgraded from handling a single lease duration to multiple lease durations.

**Describe the solution you'd like**
Pool to support the ability to use use coins on the ask side to offer a lease across multiple durations. Enabling a single ask to offer across multiple durations will be good UX while simultaneously preventing ineffective capital allocation.
aka
People will want to maximize their selling opportunities.

**Describe alternatives you've considered**
The alternative to allowing asks to offer on multiple durations is for users to speculate which single duration market would be the best alternative.

**Additional context**
The reflexive dynamic that should be avoided in Pool market design is:
It'd be undesirable for a certain market to develop as the thickest market artificially solely because it's the thickest market. E.g. which ever becomes the dominant market should be path independent of where prior bids or asks were placed.
User Journey to be avoided:
"hmm I have 1 BTC I'd like to lease out in Pool. I see durations A, B, C. C is my most preferable (say it's the longest) but everyone is participating in market B. While I'd like to offer on both C and B -- since Pool currently only supports offering on a single lease duration -- I can only submit on duration B."

The same sub optimum can evolve for bidders too.

Start by locating the order-submission flow described in the issue and reading how a single lease duration is currently selected. Trace the related Pool market and ask-order handling before defining the design. Done means one ask can offer across multiple lease durations without ineffective capital allocation, with bidder behavior considered as noted.

Written by the indexing model from the issue text.