Reduce lock levels for ALTER TABLE SET autovacuum storage options

 Reduce lock levels for ALTER TABLE SET autovacuum storage options
 
 Reduce lock levels down to ShareUpdateExclusiveLock for all autovacuum-related
 relation options when setting them using ALTER TABLE.
 
 Add infrastructure to allow varying lock levels for relation options in later
 patches. Setting multiple options together uses the highest lock level required
 for any option. Works for both main and toast tables.
 
 Fabrízio Mello, reviewed by Michael Paquier, mild edit and additional regression
 tests from myself