Make standard `SWAP` note "in-flight" fillable

[partylikeits1983]

Feature description

Background:

Currently for an Account to consume the standard SWAP note, the Account must have the REQUESTED_ASSET amount in the account's asset vault.

This is suboptimal from a liquidity point of view, since it is possible for SWAP notes to use their liquidity to fill each other.

Say there are two "complimentary" SWAP notes:

• SWAP note 1: 10 token A for 5 token B
• SWAP note 2: 2.5 token B for 5 token A

These two SWAP notes can be filled against each other using the liquidity of the other SWAP note. These two SWAP notes agree on the same price. If they did not agree on the price, the would not be complementary. Additionally, it is also possible for the sell price of one SWAP note to be lower than the buy price of another SWAP note. This would result in a potential arbitrage opportunity for the solver matching these two SWAP notes.

With the current SWAP note, the Account which consumes these two SWAP notes must have 5 tokens B and 5 tokens A, when their net asset delta will be 0 if they fill these two notes against each other.

Filling SWAP notes 1 & 2 against each other would produce the following notes:

flowchart LR
  I1[SWAP1<br/>offer 10A / want 5B] --> TX((Maker Account))
  I2[SWAP2<br/>offer 2.5B / want 5A] --> TX

  TX --> O3[SWAP1'<br/>offer 5A / want 2.5B]
  TX --> O1[P2ID: 2.5B -> SWAP1 owner]
  TX --> O2[P2ID: 5A -> SWAP2 owner]
  

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It is not necessary for the Maker Account to have any assets at all in their asset vault to fill these two SWAP notes against each other. The ability to fill SWAP notes against each other, is what I refer to as "in-flight" SWAP note filling.

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Proposal:

Make it possible for SWAP notes to be filled "in-flight` against each other. I opened a PR a while back which implemented this.

Since we are moving closer to audits, I propose we add this feature to the standard SWAP note. This adds an if / else branch to the SWAP note which checks if the consuming Account has sufficient liquidity in its asset vault to fill the SWAP note. If not, the SWAP note assumes it is being filled in the context of a batch of SWAP notes.

#1902 will need to be refactored to be updated to the latest state of next

Why is this feature needed?

The current SWAP note implementation is not flexible for solvers.

[bobbinth]

I thought we already have this solved (I remember @mmagician working on this around this time last year).