Add monitor thread for health check.

To monitor the health state of kids, We've introduced monitor
thread, which would collect the running state of each worker thread
periodically. Also we have added 4 command in Redis protocol
1. lsactivetopic: get active topics in current period
2. lsalltopic: get all topics ever existed
3. topicinfo: get inflow/outflow velocity of given topic
4. topics: get all topicinfo
All command return with JSON format.
In future development updates, we would introduce the api of RESTful
for monitoring client states and connection numbers