[PATCH 8/8] Introduce wal decoding via catalog timetravel

This introduces several things:
* applycache module which reassembles transactions from a stream of interspersed changes
* snapbuilder which builds catalog snapshots so that tuples from wal can be understood
* wal decoding into an applycache
* decode_xlog(lsn, lsn) debugging function

The applycache provides 3 major callbacks:
* apply_begin
* apply_change
* apply_commit

It is missing several parts:
- spill-to-disk
- resource usage controls
- command id handling
- passing of the correct mvcc snapshot (already has it, just doesn't pass)

The snapshot building has the most critical infrastructure but misses several
important features:
* loads of docs about the internals
* improve snapshot building/distributions
  * don't build them all the time, cache them
  * don't increase ->xmax so slowly, its inefficient
  * refcount
  * actually free them
* proper cache handling
  * we can probably reuse xl_xact_commit->nmsgs
  * generate new local inval messages from catalog changes?
* handle transactions with both ddl, and changes
  * command_id handling
  * combocid loggin/handling
* Add support for declaring tables as catalog tables that are not pg_catalog.*
* properly distribute new SnapshotNow snapshots after a transaction commits
* loads of testing/edge cases
* provision of a consistent snapshot for pg_dump
* spill state to disk at checkpoints
* xmin handling

The xlog decoding also misses several parts:
- HEAP_NEWPAGE support
- HEAP2_MULTI_INSERT support
- handling of table rewrites
---
 src/backend/replication/Makefile               |    2 +
 src/backend/replication/logical/Makefile       |   19 +
 src/backend/replication/logical/applycache.c   |  574 +++++++++++++
 src/backend/replication/logical/decode.c       |  366 +++++++++
 src/backend/replication/logical/logicalfuncs.c |  237 ++++++
 src/backend/replication/logical/snapbuild.c    | 1045 ++++++++++++++++++++++++
 src/backend/utils/time/tqual.c                 |  161 ++++
 src/include/access/transam.h                   |    5 +
 src/include/catalog/pg_proc.h                  |    3 +
 src/include/replication/applycache.h           |  239 ++++++
 src/include/replication/decode.h               |   26 +
 src/include/replication/snapbuild.h            |  119 +++
 src/include/utils/tqual.h                      |   21 +-
 13 files changed, 2816 insertions(+), 1 deletion(-)
 create mode 100644 src/backend/replication/logical/Makefile
 create mode 100644 src/backend/replication/logical/applycache.c
 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/snapbuild.c
 create mode 100644 src/include/replication/applycache.h
 create mode 100644 src/include/replication/decode.h
 create mode 100644 src/include/replication/snapbuild.h