[patch] executor and slru dtrace probes

I attached patch which was already sent on february/april, but it was
lost in time. It is originally from Robert Lor and Theo Schlossnagle.

It contains two DTrace probe groups. One is related to monitoring SLRU
and second is about executor nodes.

I merged it with the head.

Original end of mail thread is here:

http://archives.postgresql.org/pgsql-hackers/2009-04/msg00148.php

    Zdenek