SQL: Rename SQL data type DATE to DATETIME

SQL data type DATE has only the date part (e.g.: 2019-01-14)
without any time information. Previously the SQL type DATE was
referring to the ES DATE which contains also the time part along
with TZ information. To conform with SQL data types the data type
`DATE` is renamed to `DATETIME`, since it includes also the time,
as a new runtime SQL `DATE` data type will be introduced down the road,
which only contains the date part and meets the SQL standard.

Closes: elastic#36440

docs/reference/sql/endpoints/rest.asciidoc

@@ -68,7 +68,7 @@ Which returns:
         {"name": "author",       "type": "text"},
         {"name": "name",         "type": "text"},
         {"name": "page_count",   "type": "short"},
-        {"name": "release_date", "type": "date"}
+        {"name": "release_date", "type": "datetime"}
     ],
     "rows": [
         ["Peter F. Hamilton",  "Pandora's Star",       768, "2004-03-02T00:00:00.000Z"],
@@ -186,7 +186,7 @@ Douglas Adams  |The Hitchhiker's Guide to the Galaxy|180            |1979-10-12T
 In addition to the `query` and `cursor` fields, the request can
 contain `fetch_size` and `time_zone`. `fetch_size` is a hint for how
 many results to return in each page. SQL might chose to return more
-or fewer results though. `time_zone` is the time zone to use for date
-functions and date parsing. `time_zone` defaults to `utc` and can take
+or fewer results though. `time_zone` is the time zone to use for datetime
+functions and datetime parsing. `time_zone` defaults to `utc` and can take
 any values documented
 http://www.joda.org/joda-time/apidocs/org/joda/time/DateTimeZone.html[here].