Use payment_secrets in all sends in functional tests

This prepares us for requiring payment_secrets for all received
payments, by demonstrating test changes work even prior to the new
requirement.

In order to avoid needing to pipe payment secrets through to
additional places in the claim logic and then removing that
infrastructure once payment secrets are required, we use the new
payment secret storage in ChannelManager to look up the payment
secret for any given pament hash in claim and fail-back functions.
This part of the diff is reverted in the next commit.

Author: TheBlueMatt