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.