Reset the per-output-tuple exprcontext each time through the main loop in

Reset the per-output-tuple exprcontext each time through the main loop in
ExecModifyTable().  This avoids memory leakage when trigger functions leave
junk behind in that context (as they more or less must).  Problem and solution
identified by Dean Rasheed.

I'm a bit concerned about the longevity of this solution --- once a plan can
have multiple ModifyTable nodes, we are very possibly going to have to do
something different.  But it should hold up for 9.0.