sqldb: add AMP invoices SQL schema

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.

sqldb/sqlc/migrations/000002_amp_invoices.down.sql

@@ -0,0 +1,8 @@
+DROP INDEX IF EXISTS amp_htlcs_htlc_id_idx;
+DROP INDEX IF EXISTS amp_htlcs_invoice_id_idx;
+DROP INDEX IF EXISTS amp_htlcs_set_id_idx;
+DROP TABLE IF EXISTS amp_invoice_htlcs;
+
+DROP INDEX IF EXISTS amp_invoice_payments_invoice_id_idx;
+DROP TABLE IF EXISTS amp_invoice_payments;
+

sqldb/sqlc/migrations/000002_amp_invoices.up.sql

@@ -0,0 +1,54 @@
+-- amp_invoices_payments  
+CREATE TABLE IF NOT EXISTS amp_invoice_payments (
+    -- The set id identifying the payment. 
+    set_id BLOB PRIMARY KEY,
+
+    -- The state of this amp payment. This matches the state for all the htlcs 
+    -- belonging to this set id. The A in AMP stands for Atomic.
+    state SMALLINT NOT NULL,
+
+    -- Timestamp of when the first htlc for this payment was accepted.
+    created_at TIMESTAMP NOT NULL,
+
+    -- If settled, the invoice payment related to this set id.
+    settled_index INTEGER REFERENCES invoice_payments(id),
+
+    -- The invoice id this set id is related to.
+    invoice_id INTEGER NOT NULL REFERENCES invoices(id)
+);
+
+CREATE INDEX IF NOT EXISTS amp_invoice_payments_invoice_id_idx ON amp_invoice_payments(invoice_id);
+
+-- amp_invoice_htlcs contains the complementary information for an htlc related 
+-- to an AMP invoice.
+CREATE TABLE IF NOT EXISTS amp_invoice_htlcs (
+    -- The set id identifying the payment this htlc belongs to.
+    set_id BLOB NOT NULL REFERENCES amp_invoice_payments(set_id),
+
+    -- The id of the htlc this entry blongs to.
+    htlc_id BIGINT NOT NULL REFERENCES invoice_htlcs(id),
+
+    -- The invoice id this entry is related to.
+    invoice_id INTEGER NOT NULL REFERENCES invoices(id),
+
+    -- The root share for this amp htlc.
+    root_share BLOB NOT NULL,
+    
+    -- The child index for this amp htlc.
+    child_index BIGINT NOT NULL,
+
+    -- The htlc-level payment hash. An AMP htlc will carry a different payment
+    -- hash from the invoice it might be satisfying. They are needed to ensure
+    -- that we reconstruct the preimage correctly.
+    hash BLOB NOT NULL,
+    
+    -- The HTLC-level preimage that satisfies the AMP htlc's Hash.
+    -- The preimage will be derived either from secret share reconstruction of 
+    -- the shares in the AMP payload.
+    preimage BLOB 
+);
+
+CREATE INDEX IF NOT EXISTS amp_htlcs_set_id_idx ON amp_invoice_htlcs(set_id);
+CREATE INDEX IF NOT EXISTS amp_htlcs_invoice_id_idx ON amp_invoice_htlcs(invoice_id);
+CREATE INDEX IF NOT EXISTS amp_htlcs_htlc_id_idx ON amp_invoice_htlcs(htlc_id);
+