As a prerequisite for refactoring the whole PhraseSuggestionBuilder
to be able to be parsed and streamed from the coordinating node, the
DirectCandidateGenerator must implement Writeable, be able to parse
a new instance (fromXContent()) and later when transported to the
shard to generate a PhraseSuggestionContext.DirectCandidateGenerator.
Also adding equals/hashCode and tests and moving DirectCandidateGenerator
to its own DirectCandidateGeneratorBuilder class.
61c435e6a9