Add some auditing to retruned bytes in cas read

In some cases clients can be seen not getting the full content for a blob. This attempts to detect issues when streaming blobs from the server.

Aside: I also noticed we validate reading the size_bytes with digest_size.isdigit(). This can go hilariously wrong (try it yourself assert "2²".isdigit()) The only valid way in Python to check if a string is a digit, is really to try converting it and catching the ValueError.