Mini improvement: statement_cost_limit

hello ...

i picked up csaba nagy's idea and implemented a very simple yet very
useful extension.
i introduced a GUC called statement_cost_limit which can be used to
error out if a statement is expected to be too expensive.
the advantage over statement_timeout is that we are actually able to
error out before spending many seconds which is killed by
statement_timeout anyway.

    best regards,

        hans




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