Fielddata: Add doc values support to boolean fields.

This pull request makes boolean handled like dates and ipv4 addresses: things
are stored as as numerics under the hood and aggregations add some special
formatting logic in order to return true/false in addition to 1/0.

For example, here is an output of a terms aggregation on a boolean field:
```
   "aggregations": {
      "top_f": {
         "doc_count_error_upper_bound": 0,
         "buckets": [
            {
               "key": 0,
               "key_as_string": "false",
               "doc_count": 2
            },
            {
               "key": 1,
               "key_as_string": "true",
               "doc_count": 1
            }
         ]
      }
   }
```

Sorted numeric doc values are used under the hood.

Close #4678
Close #7851