dnsmgmt & AbstractSQLMgr: implement clustered dns. Also do some modifications on AbstractSQLMgr for better produce and debug.
dnsmgmt: add dns cluster notion. A cluster is a set of dns servers (master, slaves, cache) with some ACLs
AbstractSQLMgr: add new getArray function to get an array from a single SQL column in a table
AbstractSQLMgr: use dbMgr::Fetch instead of PDO::fetch in all queries"