pgsql: Per previous analysis, the most correct notion of SampleOverhead

Log Message:
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Per previous analysis, the most correct notion of SampleOverhead is that
it is just the total time to do INSTR_TIME_SET_CURRENT(), and not any of
the other code involved in InstrStartNode/InstrStopNode.  Even though I
fear we may end up reverting this patch altogether, we may as well have
the most correct version in our CVS archive.

Modified Files:
--------------
    pgsql/src/backend/executor:
        instrument.c (r1.16 -> r1.17)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/instrument.c.diff?r1=1.16&r2=1.17)