Value API: decouple value (valuenode) from HTTP context

[jknack]

• Remove usage of Context in value API
• Allow to create values without HTTP Context
• Simplify API
  • Merge ValueNode into Value
  • Merge ValueConverter and BeanConverter into Converter
  • Remove EmptyBean annotation
  • Allow Converter to get a "conversion hint" which control what type of output we need: strict, nullable, empty
  • Use method handle lookups
  • Move reflective bean converter to public package
  • Allow to override a converter including built-in converter