Replace DeleteAllKeysInList with more robust implementation

[gaul]

Over the last year we have discussed shortcomings of and improvements to DeleteAllKeysInList. Some concrete steps forward:

• [ ] parallel listing and deleting of blobs
• [ ] pluggable use of multi-blob delete
• [ ] mocked unit tests to exercise network error and timeout paths
• [ ] mechanism to evaluate liveness and progress
• [ ] retry policy tuned for billion of operations

@shrinandj and I will work on this.