For discussion PR on Fieldreference refactor WIP

I created a new class + module in logstash/util/accessor.rb for now, with its specs and Event is now using this instead of Fieldreference.

When I first started I did some work on Fieldreference + more specs, but shifted on competely refactoring into Accessors. 

When done, will probably just move it all back into Fieldreference, open to suggestions.