Replace HttpHeaders.Names/Values with HttpHeaderNames/Values

Related: 4ce994dd4fe58a03fa25b0911e5b3a2079b33692

Motivation:

In 4.1, we were not able to change the type of the HTTP header name and
value constants from String to AsciiString due to backward compatibility
reasons.

Instead of breaking backward compatibility in 4.1, we introduced new
types called HttpHeaderNames and HttpHeaderValues which provides the
AsciiString version of the constants, and then deprecated
HttpHeaders.Names/Values.

We should make the same changes while deleting the deprecated classes
activaly.

Modifications:

- Remove HttpHeaders.Names/Values and RtspHeaders
- Add HttpHeaderNames/Values and RtspHeaderNames/Values
  - Make HttpHeaderValues.WEBSOCKET lowercased because it's actually
    lowercased in all WebSocket versions but the oldest one
- Do not use AsciiString.equalsIgnoreCase(CharSeq, CharSeq) if one of
  the parameters are AsciiString
  - Avoid using AsciiString.toString() repetitively
    - Change the parameter type of some methods from String to
      CharSequence

Result:

A user who upgraded from 4.0 to 4.1 first and removed the references to
the deprecated classes and methods can easily upgrade from 4.1 to 5.0.