[RFC] txid: module for 64-bit external transaction IDs

Here's how we solved the XID indexing problem at Skype. We took
Slony-s xxid module and made it output 8-byte numbers by keeping
track of wraparound count. Thus having stable relationship
between values.

It would be good to have such functionality officially in PostgreSQL
so that all replication (and other) projects can benefit from it.
Either as a contrib module or in core.

Actually I have started porting the module into core, because
I feel the administration will be easier that way. I hoped
to post a preview of it and then finalize at Code Sprint, but
seems I won't get much time for random hacking before Summit.
Thus this post.

--
marko