EXPERIMENT - DO NOT MERGE - ability to stop path execution from a DataFetcher - #2950

This is an experiment - this is not guaranteed to be merged as is IF at all.

I wanted to explore what it might take to allow a data fetcher to return a value and say "no more processing" from here

This I think is how it could be done.

I wrote a benchmark to demonstrate a completed long list versus a list that just stops.

Benchmark                                        Mode  Cnt     Score     Error  Units
StopQueryBenchMark.benchMarkLongListThroughput  thrpt   15   182.618 ±   7.967  ops/s
StopQueryBenchMark.benchMarkStopListThroughput  thrpt   15  9537.733 ± 193.399  ops/s
StopQueryBenchMark.benchMarkLongListAvgTime      avgt   15     5.655 ±   0.198  ms/op
StopQueryBenchMark.benchMarkStopListAvgTime      avgt   15     0.104 ±   0.003  ms/op

This has some trade offs of course

• leaf node coercing DOES it happen
  • the returned object better be correct to the schema type system
• sub selection reduction does not happen
  • the returned object should fit the sub selection (and fit the type system)
• the __typename sub selection can NEVER run - since we never ran the sub selection
• instrumentation does not run for the sub selection