Re: pglogical_output - a general purpose logical decoding output plugin

* Implement relation metadata caching

* Add the relmeta_cache_size parameter for cache control

* Add an extension to get version information

* Create the pglogical_output header directory on install

* Restore 9.4 compatibility (it's small)

* Allow row filter hooks to see details of the changed tuple

* Remove forward_changesets from pglogical_output (use a hook if you want this functionality)

I'm not sure if 9.4 compat will be desirable or not. It's handy to avoid needing a separate backported version, but also confusing to do a PGXS build within a 9.6 tree against 9.4...