Deny Sender by default over Allow by default

[gusinacio]

We don't have a tap-aggregator resolver for senders so indexers must manually provide a list of sender accounts they recognize as trustful.

Currently, when a new sender decides to deposit to Escrow, tap-agent identifies it and denies it. But it makes more sense to deny all senders by default and have an "allow list" of senders with the correct tap-agent "SenderAccount" set up correctly.