get field mapping: return _boost and _analyzer by their default names also

_boost and _analyzer mapping can now be retrieved by their default name
or by given name ("path" for _analyzer, "name" for _boost)
Both still appear with their default name when just retrieving the mapping.
(see SimpleGetFieldMappingTests#testGet_boostAnd_analyzer and
     SimpleGetMappingTests#testGet_boostAnd_analyzer)

The index_name handling is removed from _boost. This never worked anyway,
see this test: https://github.com/brwe/elasticsearch/commit/36450043640f49f959d953dfd546f33606cb953a

Change in behavior:

_analyzer was never a field mapper. When defining an analyzer
in a document, the field (_analyzer or custom name) was indexed
with default string field properties. These could be overwritten by
defining an explicit mapping for this field, for example:

```
PUT testidx/doc/_mapping
{
  "_analyzer": {
    "path": "custom_analyzer"
  },
  "properties": {
    "custom_analyzer": {
      "type": "string",
      "store": true
    }
  }
}
```
Now, this explicit mapping will be ignored completely, instead
one can only set the "index" option in the definition of _analyzer
Every other option will be ignored.

Reason for this change:
The documentation says

"By default, the _analyzer field is indexed, it can be disabled by settings index to no in the mapping."

This was not true - the setting was ignored. There was a test
for the explicit definition of the mapping (AnalyzerMapperTests#testAnalyzerMappingExplicit)
but this functionallity was never documented so I assume it is not in use.

closes #7237