Backport header improvements from 5.0

Motivation:
The header class hierarchy and algorithm was improved on the master branch for versions 5.x. These improvments should be backported to the 4.1 baseline.

Modifications:
- cherry-pick the following commits from the master branch: 2374e17, 36b4157, 222d258

Result:
Header improvements in master branch are available in 4.1 branch.
Fix backward compatibility from the previous backport

Motivation:

The commit 50e06442c3f2753c9b2a506f68ea70273b829e21 changed the type of
the constants in HttpHeaders.Names and HttpHeaders.Values, making 4.1
backward-incompatible with 4.0.

It also introduces newer utility classes such as HttpHeaderUtil, which
deprecates most static methods in HttpHeaders.  To ease the migration
between 4.1 and 5.0, we should deprecate all static methods that are
non-existent in 5.0, and provide proper counterpart.

Modification:

- Revert the changes in HttpHeaders.Names and Values
- Deprecate all static methods in HttpHeaders in favor of:
  - HttpHeaderUtil
  - the member methods of HttpHeaders
  - AsciiString
- Add integer and date access methods to HttpHeaders for easier future
  migration to 5.0
- Add HttpHeaderNames and HttpHeaderValues which provide standard HTTP
  constants in AsciiString
  - Deprecate HttpHeaders.Names and Values
  - Make HttpHeaderValues.WEBSOCKET lowercased because it's actually
    lowercased in all WebSocket versions but the oldest one
- Add RtspHeaderNames and RtspHeaderValues which provide standard RTSP
  constants in AsciiString
  - Deprecate RtspHeaders.*
- 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:

Backward compatibility is recovered.  New classes and methods will make
the migration to 5.0 easier, once (Http|Rtsp)Header(Names|Values) are
ported to master.