Add functions pg_restore_relation_stats(), pg_restore_attribute_stats().

Add functions pg_restore_relation_stats(), pg_restore_attribute_stats().

Similar to the pg_set_*_stats() functions, except with a variadic
signature that's designed to be more future-proof. Additionally, most
problems are reported as WARNINGs rather than ERRORs, allowing most
stats to be restored even if some cannot.

These functions are intended to be called from pg_dump to avoid the
need to run ANALYZE after an upgrade.

Author: Corey Huinker
Discussion: https://postgr.es/m/CADkLM=eErgzn7ECDpwFcptJKOk9SxZEk5Pot4d94eVTZsvj3gw@mail.gmail.com