pgsql: Refactor to create generic WAL page read callback

Refactor to create generic WAL page read callback

Previously we didn’t have a generic WAL page read callback function,
surprisingly. Logical decoding has logical_read_local_xlog_page(), which was
actually generic, so move that to xlogfunc.c and rename to
read_local_xlog_page().
Maintain logical_read_local_xlog_page() so existing callers still work.

As requested by Michael Paquier, Alvaro Herrera and Andres Freund

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/422a55a68784fd00f4514834f3649140a9166fa5

Modified Files
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src/backend/access/transam/xlogutils.c | 166 ++++++++++++++++++++++++
src/backend/replication/logical/logicalfuncs.c | 158 +---------------------
src/include/access/xlogutils.h | 3 +
3 files changed, 172 insertions(+), 155 deletions(-)