Strict filter what clients can get approved

[msfeldstein]

Clients can submit a transaction that has an allowTrust operation that we don't notice because we only pay attention to payments. Rather than try to filter out this errant allowTrust op, we should do a whitelisting, and say only transactions that have a single payment, or one that have a single payment wrapped in the allowTrust ops that we put there (and close at the end) can be approved.