pgsql: During WAL recovery, when reading a page that we intend to

Log Message:
-----------
During WAL recovery, when reading a page that we intend to overwrite completely
from the WAL data, don't bother to physically read it; just have bufmgr.c
return a zeroed-out buffer instead.  This speeds recovery significantly,
and also avoids unnecessary failures when a page-to-be-overwritten has corrupt
page headers on disk.  This replaces a former kluge that accomplished the
latter by pretending zero_damaged_pages was always ON during WAL recovery;
which was OK when the kluge was put in, but is unsafe when restoring a WAL
log that was written with full_page_writes off.

Heikki Linnakangas

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        xlogutils.c (r1.49 -> r1.50)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlogutils.c.diff?r1=1.49&r2=1.50)
    pgsql/src/backend/storage/buffer:
        bufmgr.c (r1.216 -> r1.217)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c.diff?r1=1.216&r2=1.217)
    pgsql/src/include/storage:
        bufmgr.h (r1.102 -> r1.103)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/bufmgr.h.diff?r1=1.102&r2=1.103)