pglogical - logical replication contrib module

Hi,

I'd like to submit the replication solution which is based on the
pglogical_output [1] module (which is obviously needed for this to compile).

The pglogical contrib module provides extension which does the
master-slave logical replication based on the logical decoding.

The basic documentation is in README.md, I didn't bother making sgml
docs yet since I expect that there will be ongoing changes happening and
it's easier for me to update the markdown docs than sgml. I will do the
conversion once we start approaching committable state.

What it implements
- logical replication
- partial replication (replication sets)
- multiple sources for single subscriber
- origin filtering (so that if replication is setup both ways, there is
no cyclic replication)

It currently doesn't do multi-master or automatic DDL. I think DDL
should be relatively easy if somebody finishes the deparse extension as
the infrastructure for replicating arbitrary commands is present in this
patch.

It's rather large patch so I will just go very briefly over high level
overview of how it works, the details need to be discussed separately IMHO:
Catalogs:
  - node - stores information about "nodes" (postgresql databases)
  - node_interface - represents connection string(s) to nodes, we
separate interfaces to different catalog mainly to allow for setups
where different subscribers see different address of the provider server
  - local_node - stores exactly one tuple which points to the nodes
catalog tuple that represents the local node of the current database
  - subscription - represents subscription between two nodes, it
includes configuration of the subscription like replication set and
origin filters

Upstream:
  - basically just implements the pglogical_output hooks according to
the catalogs

Downstream:
  - several background workers
  - supervisor is worker which manages all the other workers
  - manager is per database worker which manages individual database
(node) and its subscriptions
  - apply does the actual replication, one apply process is started per
subscription, connects to the walsender on the other side and applies
the changes received from there


[1] https://commitfest.postgresql.org/8/418/

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