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.
IntelliJ IDEA warning:
Class `Frame` is made visible outside its defined scope.
Reports any references to classes which allow the class name to be used outside the class's stated scope. For instance, this inspection would report a public method which returns a private inner class, or a protected field whose type is a package-visible class. While legal Java, such references can be very confusing, and make reuse difficult.

So, as a `Frame` class defined as a `private final class Frame` and this method uses only inside current class, I think that they should be private both. Or `Frame` class should be package-local too.