wal_decoding: Introduce wal decoding via catalog timetravel

wal_decoding: Introduce wal decoding via catalog timetravel

This introduces several things:
* 'reorderbuffer' module which reassembles transactions from a stream of interspersed changes
* 'snapbuilder' which builds catalog snapshots so that tuples from wal can be understood
* logging more data into wal to facilitate logical decoding
* wal decoding into an reorderbuffer
* shared library output plugins with 5 callbacks
 * init
 * begin
 * change
 * commit
* walsender infrastructur to stream out changes and to keep the global xmin low enough
 * INIT_LOGICAL_REPLICATION $plugin; waits till a consistent snapshot is built and returns
   * initial LSN
   * replication slot identifier
   * id of a pg_export() style snapshot
 * START_LOGICAL_REPLICATION $id $lsn; streams out changes
 * uses named output plugins for output specification

Todo:
* better integrated testing infrastructure
* more frequent lowering of xmins
* more docs about the internals
* support for user declared catalog tables

Lowlevel:
* REINDEX/VACUUM FULL support for timetravel txns
* resource owner handling is suboptimal
* invalidations from uninteresting transactions (e.g. from other databases, old ones)
  need to be processed anyway
* error handling in walsender is suboptimal
* walsender slots need to be marked as catchup
* pg_receivellog needs to send a reply immediately when postgres is shutting down

Input, Testing and Review by:
Heikki Linnakangas
Kevin Grittner
Michael Paquier
Abhijit Menon-Sen
Peter Gheogegan
Robert Haas
Simon Riggs
Steve Singer

Code By:
Andres Freund

With Contributions by:
Abhijit Menon-Sen
Craig Ringer
Alvaro Herrera