remove server side constraint on unique offer ids

[turadg]

What is the Problem Being Solved?

Each offer to the smart wallet has a unique offer ID, which fulfills two requirements:

1. Offer status publishing on RPC reflects actual offers pushed. (No collisions.)
2. Maps to look up offer results (offerToUsedInvitation, offerToInvitationMakers) have a stable unique key.

To avoid keeping track of every offer id, we opted for a high water mark of a serial number:

agoric-sdk/packages/smart-wallet/src/smartWallet.js

Lines 184 to 186 in 6cdbe3f

	// To ensure every offer ID is unique we require that each is a number greater
	// than has ever been used. This high water mark is sufficient to track that.
	lastOfferId: 0,

Description of the Design

Requirement #1 is more the concern of the client, so leave it to them. If you push an offer with a colliding ID, vstorage publishing is undefined. It's easy to ensure they're unique by using UUID or a high-precision timestamp.

Requirement #2 already has a collection of ids that could collide, in the keys of those maps. If an offer will collide there, through an exception.

Security Considerations

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Test Plan

This should be backwards compatible, but do integration testing with the Wallet UI.