Sometime the broker writer failed to close, but we do not handle this failure.
This may create an empty file on remote storage but be treated as normal.
Also enhance some usabilities:
1. getting latest 2000 transactions instead of getting the earliest.
2. Show backend which download and upload tasks are being executed.
1. Apache HDFS broker support HDFS HA and Hadoop kerberos authentication.
2. New Backup and Restore function. Use Fs Broker to backup your data to HDFS or restore them from HDFS.
3. Table-Level Privileges. Grant fine-grained privileges on table-level to specified user.
4. A lot of bugs fixed.
5. Performance improvement.
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.
