Make Blobs completely immutable

[ChristopherHogan]

This involves at least 2 steps:

1. Remove the Bucket::RenameBlob function from the public API.
2. Change the behavior of a Put on an existing Blob. Previously it would destroy a Blob if it already existed, then do the Put. Instead, the Put should fail with a BLOB_EXISTS error.

Scan the API to see if any other changes are required. Adapter changes may also be required.