Remove outgoing payment accounts

[wilsonianb]

• related to Remove user accounts #156

For ☝️ accounts now only exist in Rafiki for:

• peers
• asset liquidity
• invoices
• outgoing payments

I'm wondering if there needs to be accounts for outgoing payments.
Instead of creating a tigerbeetle account and adding liquidity to it, we could send directly from the settlement account. The flow would be:

1. createPayment
2. handleQuoting
3. Rafiki notifies wallet of quote
4. Wallet gets user approval (if necessary) and reserves the quoted amount in the sender's wallet account
5. sendPayment -> Rafiki sends payment from settlement account
6. Rafiki notifies wallet of payment outcome

Not having a payment account avoids having an outstanding balance in Rafiki resulting from either a failed send or a successful fixed delivery payment.
The payment outcome notification allows the wallet to return the leftover amount to the sender or claim as fees.

[wilsonianb]

What about removing invoice accounts as well?
#191

[wilsonianb]

Opted to keep payment accounts: #197 (comment)