Statistics Publication

Adding feature in MXOSRVR server to publish three kinds of statistics info into repository: session stats, aggregation stats and query stats.
- session stats: be published when session ends;
- aggregation stats: be published every interval time during the session;
- query stats: be published when query ends. If the query doesn’t end within the limit time, query stats will be published firstly as a warning. When query ends finally, end stats will be updated.

The statistics will be populated into three tables: metric_query_table, metric_session_table and metric_query_aggr_table. User can track the historical information via a select SQL query on these tables.