Add support for multiple versions of an extension and ALTER EXTENSION UPDATE.

This follows recent discussions, so it's quite a bit different from
Dimitri's original.  There will probably be more changes once we get a bit
of experience with it, but let's get it in and start playing with it.

This is still just core code.  I'll start converting contrib modules
shortly.

Dimitri Fontaine and Tom Lane

src/include/commands/extension.h

@@ -37,6 +37,8 @@ extern Oid	InsertExtensionTuple(const char *extName, Oid extOwner,
 					 Datum extConfig, Datum extCondition,
 					 List *requiredExtensions);
 
+extern void ExecAlterExtensionStmt(AlterExtensionStmt *stmt);
+
 extern void ExecAlterExtensionContentsStmt(AlterExtensionContentsStmt *stmt);
 
 extern Oid	get_extension_oid(const char *extname, bool missing_ok);