This commit introduces a new field `invoice_msat` to the htlc_accepted
hook. If this field is specified it will replace the amount of the
invoice that belongs to the payment_hash of the HTLC on internal checks.
This is useful in scenarios where we actually expect a smaller amount
than initially specified in an invoice.
Changelog-Changed: Plugins: `htlc_accepted` hook can now override the
expected total amount of the invoice that belongs to the HTLC.
Signed-off-by: Peter Neuroth <pet.v.ne@gmail.com>