Sanitize keys which contain "." or start with "$".

A key containing "." or starting with "$" is invalid as BSON.
Add `replace_dot_in_key_with` and `replace_dollar_in_key_with` params to replace the invalid char.

### Example
Config:
```
<match ...>
  type mongo
  replace_dot_in_key_with __dot__
  replace_dollar_in_key_with __dollar__
</match>
```

Input:
```
{"foo.bar": {"$foo$bar": "baz"}}
```

The document will be inserted as the following:
```
{"foo__dot__bar": {"__dollar__foo$bar": "baz"}}
```