Add a new feature to disable even log

[taojing2002]

In this ticket, we have a steps to set up postgresql replication and rsync. After we get rid of Metacat replication, postgres replication, rsync and zookeeper will make the primary and secondary CN have the same content. In this case, the postgresql database in the secondary will be read-only. So, this cause two minor issue:

• Any read activities can't be logged into database. We decided we can ignore those read activities on the secondary cn in one of our dev meetings. So this issue was resolved.
• The Metacat instance on the secondary cn tries to log the read activities but gets an error the database can't be written (because of the postgresql replication settings).

Metacat still functions well but the second issue is annoying. So if we set a metacat property which can disable even log, the second issue will be gone.

[taojing2002]

The feature branch was reviewed and merged into the develop branch.