Previously the visibility of indexed batch mutations to open Iterators was
undocumented within the API. In practice range deletion and range key mutations
added after Iterator construction were never visible. Point operations were
immediately visible but could induce odd behavior like repeat keys when
switching directions (cockroachdb#943).

This commit defines and adopts new behavior for iteration over mutated indexed
batches. Seeking an iterator now refreshes the iterator's view of the batch,
ensuring the effects of all keys are visible. All mutations applied between
seeks are hidden from open iterators.

This interface allows iterator reuse in CockroachDB, which always seeks the
iterator after mutation. It avoids the unusual repeat-key behavior.

Fix cockroachdb#943.
Fix cockroachdb#1638.