Query DSL: Cache range filter on date field by default

A range filter on a date field with a numeric `from`/`to` value is **not** cached by default:

    DELETE /test

    PUT /test/t/1
    {
      "date": "2014-01-01"
    }

    GET /_validate/query?explain
    {
      "query": {
        "filtered": {
          "filter": {
            "range": {
              "date": {
                "from": 0
              }
            }
          }
        }
      }
    }

Returns:

    "explanation": "ConstantScore(no_cache(date:[0 TO *]))"

Closes #7114.
Actually it's a variable naming concern. Using `nowExpressionWithRounding` will make me think that the user has entered `now/d`. But, `nowExpressionWithRounding` will be true as well if user entered `2012-01-01` which is not a `now` date.

May be using `notCacheable` makes it more clear?

Still, we will use `cache = !notCacheable`.

Or if we use `cacheable` or `nowExpressionWithRounding `, we still have that negation but not at the same place:

```java
        boolean cacheable = true;
        Long lowerVal = null;
        Long upperVal = null;
        if (lowerTerm != null) {
            if (lowerTerm instanceof Number) {
                lowerVal = ((Number) lowerTerm).longValue();
            } else {
                String value = convertToString(lowerTerm);
                cacheable = !hasNowExpressionWithNoRounding(value);
                lowerVal = parseToMilliseconds(value, context, false, timeZone);
            }
        }
        if (upperTerm != null) {
            if (upperTerm instanceof Number) {
                upperVal = ((Number) upperTerm).longValue();
            } else {
                String value = convertToString(upperTerm);
                cacheable = cacheable && !hasNowExpressionWithNoRounding(value);
                upperVal = parseToMilliseconds(value, context, includeUpper, timeZone);
            }
        }

        if (explicitCaching != null) {
            if (explicitCaching) {
                cache = cacheable;
            } else {
                cache = false;
            }
        } else {
            cache = cacheable;
        }
```

WDYT?