Wrong closure_time in settlement payments

[cameel]

Currently Bankster sets closure_time in a settlement payment to the current time:

cut_off_time = get_current_utc_timestamp()

        transaction_hash = service.make_force_payment_to_provider(  # pylint: disable=no-value-for-parameter
            requestor_eth_address=requestor_ethereum_address,
            provider_eth_address=provider_ethereum_address,
            value=requestor_payable_amount,
            payment_ts=cut_off_time,
)

This is wrong. As illustrated in the #996 blueprint, the closure_time of a forced payment must be equal to payment_ts to the youngest acceptance submitted by the provider.

This is similar to the bug discovered in #1015 by @Jakub89 but this time its in the payment itself.

[PaweuB]

Pivotal:
https://www.pivotaltracker.com/story/show/162632069