tapdb: fix time zone issue with timestamps

Because sqlc by default just uses the time.Time.String() method to turn
a timestamp into the DB value, the time zone of the timestamp has an
impact as any SQL query would basically be a string comparison that
doesn't look at the time zone at all.
For example:

	WHERE a < b

Would be true for

	a = '2023-09-26 14:00:00 GMT-8' (22:00 local time)
	a = '2023-09-26 15:00:00 UTC'   (15:00 local time)

Even though 10pm is clearly after 3pm.

To fix this, we explicitly call .UTC() on any time.Time we use in any
SQL query.

The only instances where that was not yet the case in the whole tapdb
package were the newly added queries for UTXO leases.

It seems that with a later release of the sqlite driver we could
customize this behavior of how a timestamp is being formatted. But for
now the .UTC() workaround should work as well.

tapdb/assets_store.go

@@ -1682,7 +1682,7 @@ func (a *AssetStore) LeaseCoins(ctx context.Context, leaseOwner [32]byte,
 			err = q.UpdateUTXOLease(ctx, UpdateUTXOLease{
 				LeaseOwner: leaseOwner[:],
 				LeaseExpiry: sql.NullTime{
-					Time:  expiry,
+					Time:  expiry.UTC(),
 					Valid: true,
 				},
 				Outpoint: outpoint,
@@ -1991,7 +1991,7 @@ func insertAssetTransferInput(ctx context.Context, q ActiveAssetsStore,
 	return q.UpdateUTXOLease(ctx, UpdateUTXOLease{
 		LeaseOwner: finalLeaseOwner[:],
 		LeaseExpiry: sql.NullTime{
-			Time:  finalLeaseExpiry,
+			Time:  finalLeaseExpiry.UTC(),
 			Valid: true,
 		},
 		Outpoint: anchorPointBytes,

tapdb/assets_store_test.go

@@ -811,7 +811,7 @@ func TestUTXOLeases(t *testing.T) {
 	// Now we lease the first asset for 1 hour. This will cause the second
 	// one also to be leased, since it's on the same anchor transaction.
 	leaseOwner := fn.ToArray[[32]byte](test.RandBytes(32))
-	leaseExpiry := time.Now().Add(time.Hour).UTC()
+	leaseExpiry := time.Now().Add(time.Hour)
 	err = assetsStore.LeaseCoins(
 		ctx, leaseOwner, leaseExpiry, assetGen.anchorPoints[0],
 	)
@@ -825,7 +825,7 @@ func TestUTXOLeases(t *testing.T) {
 	require.Len(t, selectedAssets, 1)
 
 	// Let's now update the lease.
-	leaseExpiry = time.Now().Add(2 * time.Hour).UTC()
+	leaseExpiry = time.Now().Add(2 * time.Hour)
 	err = assetsStore.LeaseCoins(
 		ctx, leaseOwner, leaseExpiry, assetGen.anchorPoints[0],
 	)
@@ -853,7 +853,7 @@ func TestUTXOLeases(t *testing.T) {
 
 	// Update the lease again, but into the past, so that it should be
 	// removed upon cleanup.
-	leaseExpiry = time.Now().Add(-time.Hour).UTC()
+	leaseExpiry = time.Now().Add(-time.Hour)
 	err = assetsStore.LeaseCoins(
 		ctx, leaseOwner, leaseExpiry, assetGen.anchorPoints[0],
 	)