postgres_fdw: Remove SPI from postgresImportForeignStatistics.

postgres_fdw: Remove SPI from postgresImportForeignStatistics.

Previously, this function imported remote statistics by executing SQL
functions like pg_restore_relation_stats and pg_restore_attribute_stats
via SPI (in read-write mode).  As the SQL functions take a schema name
and a relation name as two separate arguments, rather than a single OID
argument, if the containing schema was concurrently renamed, the
callback function would throw an error like this:

  ERROR: schema "foo" does not exist

To fix, 1) provide new interface functions to import remote statistics
that are directly callable from FDWs and take a single OID, and 2)
modify the callback function to use the interface functions instead when
importing remote statistics.

For #1, this commit does a bit of refactoring to
relation_statistics_update and attribute_statistics_update, which are
the workhorse functions for pg_restore_relation_stats and
pg_restore_attribute_stats respectively: since they also take a schema
name and a relation name, separate the guts of them into new functions
so that they take a single OID and are callable not only from the
workhorse functions but from the interface functions introduced by #1.

Oversight in commit 28972b6fc.

Reported-by: Robert Haas <robertmhaas@gmail.com>
Suggested-by: Robert Haas <robertmhaas@gmail.com>
Author: Corey Huinker <corey.huinker@gmail.com>
Co-authored-by: Etsuro Fujita <etsuro.fujita@gmail.com>
Discussion: https://postgr.es/m/CA%2BTgmoYqMtWb4zLUkT98oFnEkJ%3DWz0Pw-ggDJrp9wnSXPzUaeQ%40mail.gmail.com
Backpatch-through: 19