Save a few cycles while creating "sticky" entries in pg_stat_statements.

Save a few cycles while creating "sticky" entries in pg_stat_statements.

There's no need to sit there and increment the stats when we know all the
increments would be zero anyway.  The actual additions might not be very
expensive, but skipping acquisition of the spinlock seems like a good
thing.  Pushing the logic about initialization of the usage count down into
entry_alloc() allows us to do that while making the code actually simpler,
not more complex.  Expansion on a suggestion by Peter Geoghegan.