graph/db+sqldb: make node migration idempotent

In this commit, the graph SQL migration is updated so that the node
migration step is retry-safe. This is done by using migration specific
logic & queries that do not use the same node-update-constraint as the
normal node upsert logic. For normal "run-time" logic, we always expect
a node update to have a newer timestamp than any previously stored one.
But for the migration, we will only ever be dealing with a single
announcement for a given node & to make things retry-safe, we dont want
the query to error if we re-insert the exact same node.

graph/db/sql_migration_test.go

@@ -155,7 +155,6 @@ func TestMigrateGraphToSQL(t *testing.T) {
 			expGraphStats: graphStats{
 				numNodes: 6,
 			},
-			expNotRetrySafety: true,
 		},
 		{
 			name: "source node",
@@ -173,7 +172,6 @@ func TestMigrateGraphToSQL(t *testing.T) {
 				numNodes:   1,
 				srcNodeSet: true,
 			},
-			expNotRetrySafety: true,
 		},
 		{
 			name:  "channel with no policies",