[Doris On ES][Bug-fix] Solve the problem of time format processing

[wuyunfeng]

#3936
Doris On ES can obtain field value from _source or docvalues:

1. From _source , get the origin value as you put, ES process indexing、docvalues for date field is converted to millisecond
2. From docvalues, before( 6.4 you get millisecond timestamp value, after(include) 6.4 you get the formatted date value :2020-06-18T12:10:30.000Z, but ES (>=6.4) provide format parameter for docvalue field request, this would coming soon for Doris On ES

After this PR was merged into Doris, Doris On ES would only correctly support to process millisecond timestamp and string format date, if you provided a seconds timestamp, Doris On ES would process wrongly which (divided by 1000 internally)

ES mapping:

{
   "timestamp_test": {
      "mappings": {
         "doc": {
            "properties": {
               "k1": {
                  "type": "date",
                  "format": "yyyy-MM-dd HH:mm:ss||yyyy-MM-dd||epoch_millis"
               }
            }
         }
      }
   }
}

ES documents:

       {
            "_index": "timestamp_test",
            "_type": "doc",
            "_id": "AXLbzdJY516Vuc7SL51m",
            "_score": 1,
            "_source": {
               "k1": "2020-6-25"
            }
         },
         {
            "_index": "timestamp_test",
            "_type": "doc",
            "_id": "AXLbzddn516Vuc7SL51n",
            "_score": 1,
            "_source": {
               "k1": 1592816393000  ->  2020/6/22 16:59:53
            }
         }

Doris Table:

CREATE EXTERNAL TABLE `timestamp_source` (
  `k1` date NULL COMMENT ""
) ENGINE=ELASTICSEARCH

enable_docvalue_scan = false

For ES 5.5:

mysql> select k1 from timestamp_source;
+------------+
| k1         |
+------------+
| 2020-06-25 |
| 2020-06-22 |
+------------+

For ES 6.5 or above:

mysql> select * from timestamp_source;
+------------+
| k1         |
+------------+
| 2020-06-25 |
| 2020-06-22 |
+------------+

enable_docvalue_scan = true

For ES 5.5:

mysql> select k1 from timestamp_dv; 
+------------+
| k1         |
+------------+
| 2020-06-25 |
| 2020-06-22 |
+------------+

For ES 6.5 or above:

mysql> select * from timestamp_dv; 
+------------+
| k1         |
+------------+
| 2020-06-25 |
| 2020-06-22 |
+------------+

[BabySid]

add a example mapping of index into this doc to show how to create a index?

[morningman]

LGTM