pgsql: Use pg_pread() and pg_pwrite() for data files and WAL.

Use pg_pread() and pg_pwrite() for data files and WAL.

Cut down on system calls by doing random I/O using offset-based OS
routines where available. Remove the code for tracking the 'virtual'
seek position. The only reason left to call FileSeek() was to get
the file's size, so provide a new function FileSize() instead.

Author: Oskari Saarenmaa, Thomas Munro
Reviewed-by: Thomas Munro, Jesper Pedersen, Tom Lane, Alvaro Herrera
Discussion: https://postgr.es/m/CAEepm=02rapCpPR3ZGF2vW=SBHSdFYO_bz_f-wwWJonmA3APgw@mail.gmail.com
Discussion: https://postgr.es/m/b8748d39-0b19-0514-a1b9-4e5a28e6a208%40gmail.com
Discussion: https://postgr.es/m/a86bd200-ebbe-d829-e3ca-0c4474b2fcb7%40ohmu.fi

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c24dcd0cfd949bdf245814c4c2b3df828ee7db36

Modified Files
--------------
src/backend/access/heap/rewriteheap.c | 2 +-
src/backend/access/transam/xlog.c | 30 +----
src/backend/storage/file/buffile.c | 46 +-------
src/backend/storage/file/fd.c | 205 +++++-----------------------------
src/backend/storage/smgr/md.c | 35 +-----
src/include/storage/fd.h | 12 +-
6 files changed, 42 insertions(+), 288 deletions(-)