Clean up code of HTTP/2 codec

Motivation:

Too many warnings from IntelliJ IDEA code inspector, PMD and FindBugs.

Modifications:

- Removed unnecessary casts, braces, modifiers, imports, throws on methods, etc.
- Added static modifiers where it is possible.
- Fixed incorrect links in javadoc.

Result:

Better code.
Only to do a canonical view. Because in `DefaultHttp2HeadersEncoder` was `Http2Exception.protocolError(...)`. I decided to add `Http2Exception.*` everywhere instead of remove it :)