If there is only one fragment of a query plan, FE will call `exec_plan_fragment` rpc to BE.
And on BE side, the `exec_plan_fragment()` will be executed directly in bthread, but it may call
some JNI method like `AttachCurrentThread()`, which will return error in bthread.
So I modify the `exec_plan_fragment` to make sure it will be executed in pthread pool.
This pr does three things:
1. add `delete_existing_files` property for outfile/export. If `delete_existing_files = true`, export/outfile will delete all files under file_path first.
2. add p2 test for export
3. modify docs
