fix(core): record correct pubkey for ln payments

[vindard]

Description

There is a bug where we optimistically assign the default lnd's pubkey to payment entries and we don't change this after it's initially set. This led to the problem where we couldn't update a payment if the pubkey we were looking up for it was false (temporarily mitigated in #4420).

This PR is to be more deliberate with recording/updating the actual pubkey that is used to send a payment as it happens thoughout the send flow.