Logical replication

- Add PUBLICATION catalogs and DDL
- Add SUBSCRIPTION catalog and DDL
- Define logical replication protocol and output plugin
- Add logical replication workers

From: Petr Jelinek <petr@2ndquadrant.com>
Reviewed-by: Steve Singer <steve@ssinger.info>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Erik Rijkers <er@xs4all.nl>
Reviewed-by: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>

doc/src/sgml/reference.sgml

@@ -54,11 +54,13 @@
    &alterOperatorClass;
    &alterOperatorFamily;
    &alterPolicy;
+   &alterPublication;
    &alterRole;
    &alterRule;
    &alterSchema;
    &alterSequence;
    &alterServer;
+   &alterSubscription;
    &alterSystem;
    &alterTable;
    &alterTableSpace;
@@ -100,11 +102,13 @@
    &createOperatorClass;
    &createOperatorFamily;
    &createPolicy;
+   &createPublication;
    &createRole;
    &createRule;
    &createSchema;
    &createSequence;
    &createServer;
+   &createSubscription;
    &createTable;
    &createTableAs;
    &createTableSpace;
@@ -144,11 +148,13 @@
    &dropOperatorFamily;
    &dropOwned;
    &dropPolicy;
+   &dropPublication;
    &dropRole;
    &dropRule;
    &dropSchema;
    &dropSequence;
    &dropServer;
+   &dropSubscription;
    &dropTable;
    &dropTableSpace;
    &dropTSConfig;