pgsql: Rework XLogReader callback system

Rework XLogReader callback system

Code review for 0dc8ead46363, prompted by a bug closed by 91c40548d5f7.

XLogReader's system for opening and closing segments had gotten too
complicated, with callbacks being passed at both the XLogReaderAllocate
level (read_page) as well as at the WALRead level (segment_open).  This
was confusing and hard to follow, so restructure things so that these
callbacks are passed together at XLogReaderAllocate time, and add
another callback to the set (segment_close) to make it a coherent whole.
Also, ensure XLogReaderState is an argument to all the callbacks, so
that they can grab at the ->private data if necessary.

Document the whole arrangement more clearly.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/20200422175754.GA19858@alvherre.pgsql

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b060dbe0001a1d6bf26cd294710f3cb203868d46

Modified Files
--------------
src/backend/access/transam/twophase.c          |   5 +-
src/backend/access/transam/xlog.c              |  10 ++-
src/backend/access/transam/xlogreader.c        |  51 ++++++-----
src/backend/access/transam/xlogutils.c         |  24 +++--
src/backend/replication/logical/logical.c      |  20 +++--
src/backend/replication/logical/logicalfuncs.c |   4 +-
src/backend/replication/slotfuncs.c            |  10 ++-
src/backend/replication/walsender.c            |  36 +++++---
src/bin/pg_rewind/parsexlog.c                  |   9 +-
src/bin/pg_waldump/pg_waldump.c                |  30 +++++--
src/include/access/xlogreader.h                | 119 ++++++++++++++++---------
src/include/access/xlogutils.h                 |   5 ++
src/include/replication/logical.h              |   4 +-
13 files changed, 214 insertions(+), 113 deletions(-)