When the public key/node ID changes, use a database with a separate namespace

[richardhuaaa]

As a starting example (may or may not be the best approach), our unit tests use this approach to create namespaced databases:

xmtpd/pkg/testutils/store.go

Lines 30 to 47 in 02c16df

	func newCtlDB(t testing.TB) (*sql.DB, string, func()) {
	return openDB(t, localTestDBDSNPrefix+localTestDBDSNSuffix)
	}
	func newInstanceDB(t testing.TB, ctx context.Context, ctlDB *sql.DB) (*sql.DB, string, func()) {
	dbName := "test_" + RandomStringLower(12)
	_, err := ctlDB.Exec("CREATE DATABASE " + dbName)
	require.NoError(t, err)
	db, dsn, cleanup := openDB(t, localTestDBDSNPrefix+"/"+dbName+localTestDBDSNSuffix)
	require.NoError(t, migrations.Migrate(ctx, db))
	return db, dsn, func() {
	cleanup()
	_, err = ctlDB.Exec("DROP DATABASE " + dbName)
	require.NoError(t, err)
	}
	}

Easiest would be to derive the namespace name from the public key, so that for rolling upgrades all new nodes roll onto the same database