This commit attempts to fix some issues with the invoice store's schema that we
couldn't foresee before the implementation was finished. This is safe as the
schema has not been instantiated yet outside of unit tests. Furthermore the
commit updates invoice store SQL queries according to fixes in the schema as
well as to prepare the higher level implementation in the upcoming commits.
Schema for AMP invocies.
AMP invoices can be paid multiple times and each payment to an AMP invoice
is identified by a `set_id`.
The A in AMP stands for `Atomic`. All the htlcs belonging to the same
AMP payment (share the same set_id) will be resolved at the same time
with the same result: settled/canceled.
AMP invoices do not have an "invoice preimage". Instead, each htcl has
its own hash/preimage. When a new htlc is added the hash for that htlc
is attached to it. When all the htlcs of a set_id have been received we
are able to compute the preimage for each one of them.
