revert to deprecated protobuf ByteSize() due to compatibility

Summary:
the change in D5386 breaks compatibility with protobuf 2.6.1, which is what we target, the newer ByteSizeLong() function being introduced in protobuf 3.1

this diff reverts that change

Test Plan:

ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5392