This change adds two contexts the execute scripts against:

* SEARCH_SCRIPT: Allows to run scripts in a search script context.
This context is used in `function_score` query's script function,
script fields, script sorting and `terms_set` query.

* FILTER_SCRIPT: Allows to run scripts in a filter script context.
This context is used in the `script` query.

In both contexts a index name needs to be specified and a sample document.
The document is needed to create an in-memory index that the script can
access via the `doc[...]` and other notations. The index name is needed
because a mapping is needed to index the document.

Examples:

```
POST /_scripts/painless/_execute
{
  "script": {
    "source": "doc['field'].value.length()"
  },
  "context" : {
    "search_script": {
      "document": {
        "field": "four"
      },
      "index": "my-index"
    }
  }
}
```

Returns:

```
{
  "result": 4
}
```

POST /_scripts/painless/_execute
{
  "script": {
    "source": "doc['field'].value.length() <= params.max_length",
    "params": {
      "max_length": 4
    }
  },
  "context" : {
    "filter_script": {
      "document": {
        "field": "four"
      },
      "index": "my-index"
    }
  }
}

Returns:

```
{
  "result": true
}
```

…jectParser

…hardAction

instead of HandledAction, because now in case score or filter contexts are used
the request needs to be redirected to a node that has an active IndexService
for the index being referenced (a node with a shard copy for that index).

…d_more_contexts

…d_more_contexts

…d_more_contexts

…d_more_contexts