Add a GUC parameter seq_page_cost, and use that everywhere we formerly

assumed that a sequential page fetch has cost 1.0.  This patch doesn't
in itself change the system's behavior at all, but it opens the door to
people adopting other units of measurement for EXPLAIN costs.  Also, if
we ever decide it's worth inventing per-tablespace access cost settings,
this change provides a workable intellectual framework for that.