[Fix SPARK-742] Task Metrics should not employ per-record timing by default

This patch does three things:

1. Makes TimedIterator a trait with two implementations (one a no-op)
2. Makes the default behavior to use the no-op implementation
3. Removes DelegateBlockFetchTracker. This is just cleanup, but it seems like
   the triat doesn't really reduce complexity in any way.

In the future we can add other implementations, e.g. ones which perform sampling.

Add zip partitions interface

(the previous commit only renamed the file)

Fix issues with streaming test cases after yarn branch merge