Its time to enable some features by default.
1. Enable FE plugins by setting `plugin_enable=true`
2. Enable dynamic partition by setting `dynamic_partition_enable=true`
3. Enable nio mysql server by setting `mysql_service_nio_enabled=true`
Also modify installation doc, add download link of MySQL client.
When starting FE with `start_fe.sh --helper xxx` command, do not allow to
point helper to FE itself. Because this is meaningless and may cause some
confusing problemes.
