How can user update an extension to a different version when BDR is active on a node?

## Describe the problem

How can user update an extension to a different version when BDR is active on a node?

```
panda=# CREATE EXTENSION pg_buffercache WITH version '1.2';
CREATE EXTENSION
panda=# ALTER EXTENSION pg_buffercache UPDATE TO '1.3';
ERROR: ALTER EXTENSION is not supported when bdr is active
```

* What are you trying to solve?

On a BDR enabled node, blocking extension updates isn't a good idea IMO. At least, BDR must support ALTER EXTENSION name UPDATE [ TO new_version ], if not others like ALTER EXTENSION .... DROP FUNCTION ...;

```
case T_AlterExtensionStmt:
/* XXX: we could support some of these */
error_unsupported_command(GetCommandTagName(CreateCommandTag(parsetree)));
break;
```