vacuumdb: Add option for analyzing only relations missing stats.

vacuumdb: Add option for analyzing only relations missing stats.

This commit adds a new --missing-stats-only option that can be used
with --analyze-only or --analyze-in-stages.  When this option is
specified, vacuumdb will analyze a relation if it lacks any
statistics for a column, expression index, or extended statistics
object.  This new option is primarily intended for use after
pg_upgrade (since it can now retain most optimizer statistics), but
it might be useful in other situations, too.

Author: Corey Huinker <corey.huinker@gmail.com>
Co-authored-by: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-by: John Naylor <johncnaylorls@gmail.com>
Discussion: https://postgr.es/m/Z5O1bpcwDrMgyrYy%40nathan