Resumable uploads have confusing account for "last committed byte"

[coryan]

In the REST-based resumable uploads we define the "last committed byte" as the value returned by the Range header of a successful chunk upload. This is the 1-based index of the highest-byte committed by the service. We use 0 to represent "nothing committed yet". And it is trivial to make a "off by one error" when converting to C++ style ranges and 0-based indexing.

In the GRPC-based resumable uploads the service returns the "committed_size", that is the number of bytes already written.

I think it would be cleaner to use this definition everywhere.

[coryan]

This cannot be done until we cleanup the mocking using internals (see #7142), and it does not affect the performance or correctness of the code, just how hard it is to maintain it. Removing from the milestone.

[coryan]

Well, to fix #7835 I need to break any existing mocks anyway. Fixing this problem at the same time (not necessarily in the same PR) seems more reasonable.