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.
Wouldn't Java compiler generate a synthetic method when a private method or constructor is invoked? I think it's better using package-local for inner class methods when they are accessed from the outer class.