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
Given that score/explainScore/setNextReader need to check whether the function is null, maybe it would make sense to keep this class but assume that the function is never null, and add a new specialization that simply holds a weight? Or alternatively, when a null function is provided in this class replace it with a function that always returns 1 as a score?