2. add 2 new proc '/current_queries' and '/current_backend_instances' to monitor the current running queries.
3. add a manual compaction api on Backend to trigger cumulative or base compaction manually.
4. add Frontend config 'max_bytes_per_broker_scanner' to limit to bytes per one broker scanner. This is to limit the memory cost of a single broker load job
5. add Frontend config 'max_unfinished_load_job' to limit load job number: if number of running load jobs exceed the limit, no more load job is allowed to be submmitted.
6. a log of bug fixed
1. rewrite metric module, add disk capacity metric
2. add Cluster load statistic proc to observer cluster load balance status
3. fix bug: show table status from db throw Nullpointer exception
4. performance: change push_down table size to 1024 in HashJoinNode
* modify modify the way of PartitionDesc analysis. fix set global variables bug
* add export job statitic proc
* merge from baidu/master
* change MASTER_CLIENT_TIMEOUT
* fix right outer join with null column
