cap withdrawals

[chadwhitacre]

Gittip's escrow is stored across multiple accounts (Balanced, New Alliance, PayPal, cold storage etc.; see #1988, whence this is reticketed). We see spikes in withdrawals when someone accumulates money in Gittip over a year and then connects a funding sink. It takes time to move money between those accounts, so, while we always have everyone's full escrowed amount available within our escrow as a whole, we do run the risk of not having someone's money available right now for a particular sink type.

To mitigate this risk, we should cap withdrawals. The algorithm should be something like:

• Reduce the amount of the top withdrawer in a batch as needed, down to the amount of the second-highest withdrawer.
• If that's not enough, reduce the amount of the first- and second-highest withdrawers down to the amount of the third-highest. Etc.

We should do this similarly for both Balanced and PayPal.

We should mention this in the note for the exchange.

We should ensure that this interacts with escrow rebalancing (#1092) in such a way that users are only affected by this cap for a week or so.

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[chadwhitacre]

Closing in light of our decision to shut down Gratipay.

Thank you all for a great run, and I'm sorry it didn't work out! 😞 💃