Clean up and simplify Go pipeline example.

[matttproud]

This commit cleans up a few things about the Go pipeline:

1. idiom: use sync.WaitGroup instead of atomic counter for
   rendezvous.
2. throughput: switch to buffered channels since message handout
   needn't be synchronous.
3. cruft removal: removed dead/non-used functions.
4. throughput: kick off reading from all buffers in separate Go
   routines since the scheduler will self-bound their operation
   per the maximum processors.
5. simplify/throughput: drop pre-processing of tallies into maps
   and replace it with a channel that merely contains the
   neighborhood name. Neighborhood processing occurs in-flight
   with the mappers.
6. throughput: the regexp/substring match can be made more
   efficient by prima facie rejecting candidates whose values are
   too short to contain the needle.
7. readability: separate mapper and reducers into individual
   routines.
8. tidying: remedy lint errors.

Overall this change is performance neutral to being an improvement.
In my local runs, I was able to shave off about 2.5 seconds on the
substring run with this.

[dimroc]

Great changes. Thanks! 👍