pgsql: Teach xlogreader to follow timeline switches

Teach xlogreader to follow timeline switches

Uses page-based mechanism to ensure we’re using the correct timeline.

Tests are included to exercise the functionality using a cold disk-level copy
of the master that's started up as a replica with slots intact, but the
intended use of the functionality is with later features.

Craig Ringer, reviewed by Simon Riggs and Andres Freund

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/1148e22a82edc96172fc78855da392b6f0015c88

Modified Files
--------------
src/backend/access/transam/xlogutils.c | 215 +++++++++++++++++++--
src/backend/replication/logical/logicalfuncs.c | 8 +-
src/backend/replication/walsender.c | 11 +-
src/include/access/xlogreader.h | 16 ++
src/include/access/xlogutils.h | 3 +
src/test/recovery/Makefile | 2 +
.../recovery/t/010_logical_decoding_timelines.pl | 130 +++++++++++++
7 files changed, 365 insertions(+), 20 deletions(-)