pgsql: Speed up CREATE DATABASE by deferring the fsyncs until after

Log Message:
-----------
Speed up CREATE DATABASE by deferring the fsyncs until after copying
all the data and using posix_fadvise to nudge the OS into flushing it
earlier. This also hopefully makes CREATE DATABASE avoid spamming the
cache.

Tests show a big speedup on Linux at least on some filesystems.

Idea and patch from Andres Freund.

Modified Files:
--------------
    pgsql/src/backend/storage/file:
        fd.c (r1.153 -> r1.154)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/file/fd.c?r1=1.153&r2=1.154)
    pgsql/src/include/storage:
        fd.h (r1.66 -> r1.67)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/fd.h?r1=1.66&r2=1.67)
    pgsql/src/port:
        copydir.c (r1.25 -> r1.26)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/port/copydir.c?r1=1.25&r2=1.26)