Properly handle Signers vs. Senders

[aasseman]

Senders:

• Control their Escrow account
• Add and remove signing addresses to their escrow account

Signers:

• Sign Receipts and RAVs
• There can be multiple signers per Sender/Escrow account
• Signers can be revoked (\~1 month thawing)

tl;dr (pseudo code or something):

Sender {
  escrow_account: EscrowAccount
  signers: [Address]
}