3448: code review comments.

3448: code review comments.

Extend GetBlock() to optionally update the file modification time, so
PUT operations can update the timestamp of an existing block.

UnixVolume.Delete() returns nil if the file is too new to delete (the
reasoning here is that this is the correct thing for the server to do,
even if the result technically does not fulfill the user's request, so
the server should return success).

Refs #3448.