httputil: Deprecate some args to HTTPServerRequest constructor

httputil: Deprecate some args to HTTPServerRequest constructor

HTTPServerRequest has some redundant and/or obsolete constructor
arguments. #3542 cleaned this up a bit, but it included a backwards
incompatible change with no deprecation warning. This change adds
deprecation warnings in anticipation of deleting the old arguments
in Tornado 6.7.

In Tornado 6.5, all arguments were officially optional, but things would
only partially work without the method and uri arguments. #3542 made
either the uri or start_line arguments mandatory.

This change makes the method and uri arguments deprecated. It is also
deprecated to use method and uri at the same time as start_line
(previously method and uri would be silently ignored). In Tornado 6.7,
the method, uri, and version arguments will be removed, and start_line
will be required.

Updates #3500
Updates #3542
Fixes #3637