[PATCH 11/14] 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:
* testing infrastructure (isolationtester)
* persistence/spilling to disk of built snapshots, longrunning
  transactions
* user docs
* more frequent lowering of xmins
* more docs about the internals
* support for user declared catalog tables
* actual exporting of initial pg_export snapshots after
  INIT_LOGICAL_REPLICATION
* own shared memory segment instead of piggybacking on walsender's
* nicer interface between snapbuild.c, reorderbuffer.c, decode.c and the
  outside.
* more frequent xl_running_xid's so xmin can be upped more frequently
* add STOP_LOGICAL_REPLICATION $id
---
 src/backend/access/heap/heapam.c                |  280 +++++-
 src/backend/access/transam/xlog.c               |    1 +
 src/backend/catalog/index.c                     |   74 ++
 src/backend/replication/Makefile                |    2 +
 src/backend/replication/logical/Makefile        |   19 +
 src/backend/replication/logical/decode.c        |  496 ++++++++++
 src/backend/replication/logical/logicalfuncs.c  |  247 +++++
 src/backend/replication/logical/reorderbuffer.c | 1156 +++++++++++++++++++++++
 src/backend/replication/logical/snapbuild.c     | 1144 ++++++++++++++++++++++
 src/backend/replication/repl_gram.y             |   32 +-
 src/backend/replication/repl_scanner.l          |    2 +
 src/backend/replication/walsender.c             |  566 ++++++++++-
 src/backend/storage/ipc/procarray.c             |   23 +
 src/backend/storage/ipc/standby.c               |    8 +-
 src/backend/utils/cache/inval.c                 |    2 +-
 src/backend/utils/cache/relcache.c              |    3 +-
 src/backend/utils/misc/guc.c                    |   11 +
 src/backend/utils/time/tqual.c                  |  249 +++++
 src/bin/pg_controldata/pg_controldata.c         |    2 +
 src/include/access/heapam_xlog.h                |   23 +
 src/include/access/transam.h                    |    5 +
 src/include/access/xlog.h                       |    3 +-
 src/include/catalog/index.h                     |    4 +
 src/include/nodes/nodes.h                       |    2 +
 src/include/nodes/replnodes.h                   |   22 +
 src/include/replication/decode.h                |   21 +
 src/include/replication/logicalfuncs.h          |   44 +
 src/include/replication/output_plugin.h         |   76 ++
 src/include/replication/reorderbuffer.h         |  284 ++++++
 src/include/replication/snapbuild.h             |  128 +++
 src/include/replication/walsender.h             |    1 +
 src/include/replication/walsender_private.h     |   34 +-
 src/include/storage/itemptr.h                   |    3 +
 src/include/storage/sinval.h                    |    2 +
 src/include/utils/tqual.h                       |   31 +-
 35 files changed, 4966 insertions(+), 34 deletions(-)
 create mode 100644 src/backend/replication/logical/Makefile
 create mode 100644 src/backend/replication/logical/decode.c
 create mode 100644 src/backend/replication/logical/logicalfuncs.c
 create mode 100644 src/backend/replication/logical/reorderbuffer.c
 create mode 100644 src/backend/replication/logical/snapbuild.c
 create mode 100644 src/include/replication/decode.h
 create mode 100644 src/include/replication/logicalfuncs.h
 create mode 100644 src/include/replication/output_plugin.h
 create mode 100644 src/include/replication/reorderbuffer.h
 create mode 100644 src/include/replication/snapbuild.h