Reduce DEBUG level of catcache refreshing messages

When testing extensions using pgregress, it can be useful to introduce
some new DEBUG logs which are specific to the extension and change the
log level during part of the of the test.

There's a problem though: Often a "rehashing catalog cache ..." debug
message will also show up in those cases. It's not always possible to
predict when these messages show, and when they do their contents can
easily change if changes are made to an unrelated test or when run
against a different Postgres version. This change lowers the log level
of these messages to DEBUG5, so that they can be ignored while still
showing other (more predictable) DEBUG messages.