Fix deprecated ByteSize() for protobuf >= 3.1

Summary:
Based on the reverted D5386 but accounting on protobuf version to remain
compatible.
Note that autotools will still have the warning.

Test Plan:
With protobuf >= 3.1:

ninja check

Make sure the warning is gone.

Run the gitian builds.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6275