function_score: add optional weight parameter per function

Weights can be defined per function like this:

```
"function_score": {
    "functions": [
        {
            "filter": {},
            "FUNCTION": {},
            "weight": number
        }
        ...
```
If `weight` is given without `FUNCTION` then `weight` behaves like `boost_factor`.
This commit deprecates `boost_factor`.

The following is valid:

```
POST testidx/_search
{
  "query": {
    "function_score": {
      "weight": 2
    }
  }
}
POST testidx/_search
{
  "query": {
    "function_score": {
      "functions": [
        {
          "weight": 2
        },
        ...
      ]
    }
  }
}
POST testidx/_search
{
  "query": {
    "function_score": {
      "functions": [
        {
          "FUNCTION": {},
          "weight": 2
        },
        ...
      ]
    }
  }
}
POST testidx/_search
{
  "query": {
    "function_score": {
      "functions": [
        {
          "filter": {},
          "weight": 2
        },
        ...
      ]
    }
  }
}
POST testidx/_search
{
  "query": {
    "function_score": {
      "functions": [
        {
          "filter": {},
          "FUNCTION": {},
          "weight": 2
        },
        ...
      ]
    }
  }
}
```

The following is not valid:

```
POST testidx/_search
{
  "query": {
    "function_score": {
      "weight": 2,
      "FUNCTION(including boost_factor)": 2
    }
  }
}

POST testidx/_search
{
  "query": {
    "function_score": {
      "functions": [
        {
          "weight": 2,
          "boost_factor": 2
        }
      ]
    }
  }
}
````

closes #6955
I think it is a bit weird that if you want to serialize a ScoreFunctionBuilder, you need to call both toXContent and buildWeight. Maybe we could rather have something like:

```java
  protected void buildWeight(XContentBuilder builder) throws IOException {
    if (weight != null) {
      builder.field(FunctionScoreQueryParser.WEIGHT_FIELD.getPreferredName(), weight);
    }
  }

  @Override
  public final void toXContent(XContentBuilder builder, Params params) {
    buildWeight(builder);
    doXContent(builder, params);
  }

  protected abstract void doXContent(XContentBuilder builder, Params params) throws IOException;
```

This way, toXContent would be all that needs to be called to serialize a ScoreFunction and the functions that want to override buildWeight could still do it?