to make them cancelable. Also,
* rename BulkDelete->BulkDeleteAll
this leaves room for a new method `BulkDelete(items storage.Items)` that
does a bulk deletion of a specified list of items, as opposed to
deleting _everything_. such a method would be used in the
`cleanupItems()` function found in utils.go, because when individual
deletes are fairly slow, that step takes way too long during tests.
* use BulkDelete method if available
nothing currently provides `BulkDelete(items storage.Items) error`,
but we made use of it with the Bigtable testing and code, and may make
use of it again when adding new kv backends.
* and eliminate the global context in test_iterate.go
Change-Id: I171c7a3818beffbad969b131e98b9bbe3f324bf2
94651921c3