Currently, Doris only supports loading data into one table in a load execution plan.
Which means if user want to load the data from a file to, for example, 4 tables, it has to
generate 4 load execution plans, and each plan will read the data from file alone.

It is inefficient. So I would like to implement a new load execution plan which contains 2
new plan nodes:

1. DataDuplicateNode
   DataDuplicateNode will get data from its child (such as BrokerScanNode), and duplicate
   these data for target tuples.
2. MultiOlapTableSink
   MultiOlapTableSink is a DataSink, which contains several OlapTableSink.
   It should be the sink of DataDuplicateNode.

The plan fragment should looks like:

+--------------------------+
|PlanFragment              |
|                          |
|    +-------------------+ |
|    |MultiOlapTableSink | |
|    |                   | |
|    |   OlapTableSink +-----> TabletsChannel
|    |                   | |
|    |   OlapTableSink +-----> TabletsChannel
|    |                   | |
|    |   OlapTableSink +-----> TabletsChannel
|    +---------^---------+ |
|              |           |
|              | 2. Send   |
|              |           |
|     +--------+--------+  |
|     |                 |  |
|     |DataDuplicateNode|  |
|     |                 |  |
|     +--------+--------+  |
|              |           |
|              | 1. GetNext|
|              |           |
|     +--------v--------+  |
|     |                 |  |
|     |  BrokerScanNode |  |
|     |                 |  |
|     +-----------------+  |
+--------------------------+