Handle heap rewrites even better in logical decoding

 Handle heap rewrites even better in logical decoding
 
 Logical decoding should not publish anything about tables created as
 part of a heap rewrite during DDL.  Those tables don't exist externally,
 so consumers of logical decoding cannot do anything sensible with that
 information.  In ab28feae2bd3d4629bd73ae3548e671c57d785f0, we worked
 around this for built-in logical replication, but that was hack.
 
 This is a more proper fix: We mark such transient heaps using the new
 field pg_class.relwrite, linking to the original relation OID.  By
 default, we ignore them in logical decoding before they get to the
 output plugin.  Optionally, a plugin can register their interest in
 getting such changes, if they handle DDL specially, in which case the
 new field will help them get information about the actual table.
 
 Reviewed-by: Craig Ringer <craig@2ndquadrant.com>