Demote log lines to improve debug log SNR.

The debug logging is verbose and sometimes hard to read.

Demote some of the boilerplate output to log level 9, to make it easier
to see the important parts on lower verbosity levels.

src/audit_log/audit_log.cc

@@ -276,7 +276,7 @@ bool AuditLog::saveIfRelevant(Transaction *transaction, int parts) {
     if ((m_status == RelevantOnlyAuditLogStatus
         && this->isRelevant(transaction->m_httpCodeReturned) == false)
         && saveAnyway == false) {
-        transaction->debug(5, "Return code `" +
+        transaction->debug(9, "Return code `" +
             std::to_string(transaction->m_httpCodeReturned) + "'" \
             " is not interesting to audit logs, relevant code(s): `" +
             m_relevant + "'.");