MXS-2592 Add configuration for session specific in-memory log

When enabled each session will write log messages in the in-memory log.
If session ends in error this log is written to the actual log in disk.

server/modules/protocol/MySQL/mariadbclient/mysql_client.cc

@@ -1585,6 +1585,11 @@ static int gw_client_hangup_event(DCB* dcb)
                 session_dump_statements(session);
             }
 
+            if (session_get_session_trace())
+            {
+                session_dump_log(session);
+            }
+
             // The client did not send a COM_QUIT packet
             std::string errmsg {"Connection killed by MaxScale"};
             std::string extra {session_get_close_reason(dcb->session)};