pglogical_output - a general purpose logical decoding output plugin

Hi all

I'd like to submit pglogical_output for inclusion in the 9.6 series as
a contrib.

The output plugin is suitable for a number of uses. It's designed
primarily to supply a data stream to drive a logical replication
client running in another PostgreSQL instance, like the pglogical
client discussed at PGConf.EU 2015. However, the plugin can also be
used to drive other replication solutions, message queues, etc.

This isn't an extension, in that it doesn't actually provide any
extension control file, and you can't CREATE EXTENSION it. It's only
usable via the logical decoding interfaces - at the SQL level, or via
the walsender.

See the README.md and DESIGN.md in the attached patch for details on
the plugin. I will follow up with a summary in a separate mail, along
with a few points I'd value input on or want to focus discussion on.

I anticipate that I'll be following up with a few tweaks, but at this
point the plugin is feature-complete, documented and substantially
ready for inclusion as a contrib.

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