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.
I think OutboundFlowState should be private as well.  I suspect that should fix the warning ... can you make that change.  Hopefully it doesn't break any tests :)