After tablet level metrics is supported, the http metrics API may response
a very large body when a BE holds a large number of tablets, and cause heavy
network traffic.
This patch introduce http content compression to reduce network traffic.
1. Implement Backend http server using libevent instead of mongoose.
2. Remove Old Hypertable rpc framework, use brpc instead.
3. Change rpc from FE to BE to brpc.
4. Fs broker support HDFS HA.
5. add more metrics to monitor.
6. Lots of bug fixed.
