raising the default prepareTheshold

I'd like to raise the default prepareThreshold to some non-zero value,
perhaps 5 or so. This means that PreparedStatements that are used more
than 5 times will allocate and use a named backend statement.

Usually, using a named statement is a performance win as it avoids
reparsing and replanning the query on each execution; I've seen
measurable improvement (at least 15%, from memory) on heavily-reused
simple queries where the parse/plan cost is a noticable fraction of
total execution time.

However, with a 8.0 backend, if the planner benefits from knowing the
particular parameter values in use for a particular query, then using an
unnamed statement may be faster due to using an improved plan. Under 7.4
there will be no difference; both named and unnamed statements will use
the generic plan.

So using named statements always by default (default prepareThreshold =
1) probably isn't a good idea. Using a default prepareThreshold > 1
seems like a reasonable compromise.

What does everyone think?

-O