Use pread and pwrite instead of lseek + write and read

Hi,

The Uber blog post, among other things, pointed out that PG uses lseek +
read instead of pread. I didn't see any discussion around that and my
Google searches didn't find any posts about pread / pwrite for the past
10 years.

With that plus the "C++ port" thread in mind, I was wondering if it's
time to see if we could do better by just utilizing newer C and POSIX
features.

The attached patch replaces FileWrite and FileRead with FileWriteAt and
FileReadAt and removes most FileSeek calls. FileSeek is still around so
we can find the end of a file, but it's not used for anything else.

On my laptop a simple pgbench run (scale 100, 15 minutes) shows a 1.5%
performance improvement. A 1.5% performance improvement is small but
measurable - and IMV more importantly it allows us to drop more than 100
lines of backwards (compatible?) code; maybe we could start targeting
more recent platforms in v10?

Obviously this patch needs some more work before it could be merged, and
we probably still need a fallback for some platforms without pread and
pwrite (afaik Windows doesn't implement them.)

/ Oskari