Vulnerability Instance

Tornado vulnerable to HTTP request smuggling via improper parsing of `Content-Length` fields and chunk lengths
## Summary
Tornado interprets `-`, `+`, and `_` in chunk length and `Content-Length` values, which are not allowed by the HTTP RFCs. This can result in request smuggling when Tornado is deployed behind certain proxies that interpret those non-standard characters differently. This is known to apply to older versions of haproxy, although the current release is not affected.

## Details
Tornado uses the `int` constructor to parse the values of `Content-Length` headers and chunk lengths in the following locations:
### `tornado/http1connection.py:445`
```python3
      self._expected_content_remaining = int(headers["Content-Length"])
```
### `tornado/http1connection.py:621`
```python3
        content_length = int(headers["Content-Length"]) # type: Optional[int]
```
### `tornado/http1connection.py:671`
```python3
      chunk_len = int(chunk_len_str.strip(), 16)
```
Because `int("0_0") == int("+0") == int("-0") == int("0")`, using the `int` constructor to parse and validate strings that should contain only ASCII digits is not a good strategy.