pgsql: Read WAL directly from WAL buffers.

Read WAL directly from WAL buffers.

If available, read directly from WAL buffers, avoiding the need to go
through the filesystem. Only for physical replication for now, but can
be expanded to other callers.

In preparation for replicating unflushed WAL data.

Author: Bharath Rupireddy
Discussion: https://postgr.es/m/CALj2ACXKKK%3DwbiG5_t6dGao5GoecMwRkhr7GjVBM_jg54%2BNa%3DQ%40mail.gmail.com
Reviewed-by: Andres Freund, Alvaro Herrera, Nathan Bossart, Dilip Kumar, Nitin Jadhav, Melih Mutlu, Kyotaro Horiguchi

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/91f2cae7a4e664e9c0472b364c7db29d755ab151

Modified Files
--------------
src/backend/access/transam/xlog.c       | 120 ++++++++++++++++++++++++++++++++
src/backend/access/transam/xlogreader.c |   3 -
src/backend/replication/walsender.c     |  12 +++-
src/include/access/xlog.h               |   3 +
4 files changed, 134 insertions(+), 4 deletions(-)