1. Change word of palo to doris in conf file.
2. Set default meta_dir to ${DORIS_HOME}/doris-meta
3. Comment out FE meta_dir, leave it to ${DORIS_HOME}/doris-meta, as exsting in FE Config.java.
4. Comment out BE storage_root_path, leave it to ${DORIS_HOME}/storage, as exsting in BE config.h.
NOTICE: default config is changed.
1. ix bugs in query layer.
2. remove some redundant code in BE
3. support specify multi helper node when starting FE
4. add proc 'cluster_load_statistic' to show load balance situation of Palo
Add new features:
1. plugins of Ambari and k8s deploy
2. specified config 'priority_network' to solve some ip problems
Fix bugs:
fix bugs that rebalance does not work in some case.
fix count(*) from union stmt bug
fix some union stmt bugs
fix bugs when try to schema change a clone replica
