Fix rare failure to invalidate relation cache init file (Tom Lane)

With just the wrong timing of concurrent activity, a VACUUM FULL on a system catalog might fail to update the “init file” that's used to avoid cache-loading work for new sessions. This would result in later sessions being unable to access that catalog at all. This is a very ancient bug, but it's so hard to trigger that no reproducible case had been seen until recently.

Avoid deadlock between incoming sessions and CREATE/DROP DATABASE (Tom Lane)

A new session starting in a database that is the target of a DROP DATABASE command, or is the template for a CREATE DATABASE command, could cause the command to wait for five seconds and then fail, even if the new session would have exited before that.