Improve DELETE functionality.

[abbashus]

Currently, the SQL DELETE query does not give proper results to the user.

Data:

PUT bank/_doc/1
{
  "name" : "Abbas",
  "state" : "WA"
}

PUT bank/_doc/2
{
  "name" : "John",
  "state" : "CA" 
}

Vanilla DELETE query:

POST _opendistro/_sql/_explain?
{
    "query" : "DELETE FROM bank" 
}

==== Explain: ====
{"size":1000,"query":{"match_all":{"boost":1.0}},"_source":false}

==== ES raw output: ====
{
  "took" : 84,
  "timed_out" : false,
  "total" : 2,
  "updated" : 0,
  "created" : 0,
  "deleted" : 2, 
  "batches" : 1,
  "version_conflicts" : 0,
  "noops" : 0,
  "retries" : {
    "bulk" : 0,
    "search" : 0
  },
  "throttled_millis" : 0,
  "requests_per_second" : -1.0,
  "throttled_until_millis" : 0,
  "failures" : [ ]
}

==== _opendistro/_sql?_format=jdbc ====
{
  "error": {
    "reason": "There was internal problem at backend",
    "details": "class com.amazon.opendistroforelasticsearch.sql.domain.Delete cannot be cast to class com.amazon.opendistroforelasticsearch.sql.domain.Select (com.amazon.opendistroforelasticsearch.sql.domain.Delete and com.amazon.opendistroforelasticsearch.sql.domain.Select are in unnamed module of loader java.net.FactoryURLClassLoader @46963479)",
    "type": "ClassCastException"
  },
  "status": 500
}

==== _opendistro/_sql?format=csv ====
null 

DELETE query with condition(s):

POST _opendistro/_sql/_explain?
{
    "query" : "DELETE FROM bank WHERE name = 'Abbas' " 
}

{
  "size": 1000,
  "query": {
    "bool": {
      "must": [
        {
          "term": {
            "name": {
              "value": "Abbas",
              "boost": 1
            }
          }
        }
      ],
      "adjust_pure_negative": true,
      "boost": 1
    }
  },
  "_source": false
}

Observation:

• Even though the actual data is deleted , output of format=jdbc emits exception and for format=csv it just outputs null which is not very informative.
• For DELETE query with WHERE clause, for fields with type text + keyword, the ES DSL is incorrect and thus data is not deleted as expected. It should be name.keyword instead of name in Term query.

Proposed Solution:
For format=jdbc, result should be something like this:

{
  "schema": [
    {
      "name": "deleted_rows",
      "type": "text"
    }
  ],
  "total": 1,
  "datarows": [
    [
      2
    ]
  ],
  "size": 1,
  "status": 200
}

For format=csv, result should be something like this to be consistent with jdbc.

deleted_rows
2

For incorrect ES DSL , make it go through TermFieldRewriter.

[abbashus]

Closing #96 in lieu of this.

[dai-chen]

The DELETE for JDBC (default) format has been fixed. Will create separate issue if CSV needs this support.