Logical Replication Helpers WIP for discussion

Hello,

we've made few helper functions for making logical replication easier, I
bundled it into contrib module as this is mainly for discussion at this
time (I don't expect this to get committed any time soon, but it is good
way to iron out protocol, etc).

I created sample logical decoding plugin that uses those functions and
which can be used for passing DML changes in platform/version
independent (hopefully) format.

I will post sample apply BG worker also once I get some initial feedback
about this.

It's hard to write tests for this as the binary changes contain
transaction ids and timestamps so the data changes constantly.

This is of course based on the BDR work Andres, Craig and myself have
been doing.

--
Petr Jelinek http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services