Improve publicAuction.js

[dtribble]

What is the Problem Being Solved?

publicAuction was written to much earlier APIs, and so is not providing good example of current API usage.

• The code is too complicated for the function it's providing (121 lines of code)
• It's working too hard to check things it shouldn't have to care about
• The termination is based on number of bids, which is not part of any interesting use case
• auction.js has helper functions just for it. Since htey are specialized to particular Keywords, they should just be private in the module with publicAuction.

Description of the Design

• Redo using the affordances of the new APIS
• Offer checking should rely on Zoe or use helpers
• Termination should at the very least be "whenever the seller closes the auction" since that is at least simpler.
• Add notifiers?
• It could incrementally update the auction state on each bid

Security Considerations
The refundAmount logic is a hazard and should not be necessary. It's implied by the transfer, and so linear operations would be better (e.g., amount.split).

Test Plan
Tests should stay the same except that auction-close should use a better mechanism

[katelynsills]

I'm going to capture the other comments here:

#1414 (comment)
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#1414 (comment)
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#1414 (comment)

[katelynsills]

@dtribble: I didn't understand this part: The refundAmount logic is a hazard.