[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