planner & mpp: need a explicit MPPPartitionType to mark PassThrough Exchange.

[LittleFall]

Enhancement

Currently, ExchangeSender/Receiver has an exchange type PassThrough, It corresponds to AnyType in MPPPartitionType. But AnyType may also mean Don't exchange, which creates several inconveniences; For example:

1. if we want to do a passthrough exchange, we must explicitly call the function enforceExchangerImpl, instead of guaranteeing it by enforceProperty.
2. AnyType of MPPPartitionType is ambiguous, it may represent an exchange that does not require an exchange, or it may represent an exchange that requires an exchange.
3. It may bring difficulties for new operators to push down, such as window function push down, it may need a sort after passthrough exchange.
4. AnyType can't reuse the property, such as this sql will do an extra passthrough.

mysql>  explain select count(distinct a) from (select count(distinct deptid) a from employee) x;
+----------------------------------------------+---------+-------------------+----------------+------------------------------------------------------+
| id                                           | estRows | task              | access object  | operator info                                        |
+----------------------------------------------+---------+-------------------+----------------+------------------------------------------------------+
| TableReader_51                               | 1.00    | root              |                | data:ExchangeSender_50                               |
| └─ExchangeSender_50                          | 1.00    | batchCop[tiflash] |                | ExchangeType: PassThrough                            |
|   └─Projection_46                            | 1.00    | batchCop[tiflash] |                | Column#6                                             |
|     └─HashAgg_47                             | 1.00    | batchCop[tiflash] |                | funcs:count(distinct Column#5)->Column#6             |
|       └─ExchangeReceiver_49                  | 1.00    | batchCop[tiflash] |                |                                                      |
|         └─ExchangeSender_48                  | 1.00    | batchCop[tiflash] |                | ExchangeType: PassThrough                            |
|           └─HashAgg_45                       | 1.00    | batchCop[tiflash] |                | group by:Column#5,                                   |
|             └─Projection_35                  | 1.00    | batchCop[tiflash] |                | Column#5                                             |
|               └─HashAgg_36                   | 1.00    | batchCop[tiflash] |                | funcs:count(distinct test.employee.deptid)->Column#5 |
|                 └─ExchangeReceiver_38        | 1.00    | batchCop[tiflash] |                |                                                      |
|                   └─ExchangeSender_37        | 1.00    | batchCop[tiflash] |                | ExchangeType: PassThrough                            |
|                     └─HashAgg_34             | 1.00    | batchCop[tiflash] |                | group by:test.employee.deptid,                       |
|                       └─TableFullScan_19     | 17.00   | batchCop[tiflash] | table:employee | keep order:false, stats:pseudo                       |
+----------------------------------------------+---------+-------------------+----------------+------------------------------------------------------+
13 rows in set (0.00 sec)

[LittleFall]

cc @fzhedu @windtalker @winoros