pgsql: Add hooks to allow debugging and performance measurement plugins

Log Message:
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Add hooks to allow debugging and performance measurement plugins
to instrument PL/pgSQL. Korry Douglas

Modified Files:
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pgsql/src/pl/plpgsql/src:
pl_exec.c (r1.175 -> r1.176)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.175&r2=1.176)
pl_handler.c (r1.30 -> r1.31)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_handler.c.diff?r1=1.30&r2=1.31)
plpgsql.h (r1.79 -> r1.80)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h.diff?r1=1.79&r2=1.80)