graph/db+sqldb: make channel SQL mig retry-safe

In this commit, we make the channel part of the graph SQL migration
idempotent (retry-safe!). We do this by adding a migration-only channel
insert query that will not error out if a the query is called and a
chanenl with the given scid&version already exists. We also ensure that
errors are not thrown if existing channel features & extra types are
re-added.

graph/db/sql_migration_test.go

@@ -225,7 +225,6 @@ func TestMigrateGraphToSQL(t *testing.T) {
 				numNodes:    4,
 				numChannels: 3,
 			},
-			expNotRetrySafety: true,
 		},
 		{
 			name:  "channels and policies",