httputil: Improve handling of trailing whitespace in headers

httputil: Improve handling of trailing whitespace in headers

HTTPHeaders had undocumented assumptions about trailing whitespace,
leading to an unintentional regression in Tornado 6.4.1 in which
passing the arguments of an AsyncHTTPClient header_callback to
HTTPHeaders.parse_line would result in errors.

This commit moves newline parsing from parse to parse_line.
It also strips trailing whitespace from continuation lines (trailing
whitespace is not allowed in HTTP headers, but we didn't reject it
in continuation lines).

This commit also deprecates continuation lines and the legacy
handling of LF without CR.

Fixes #3321