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