clients/horizonclient+txnbuild: Update CHANGELOGs for horizonclient 4.0 release#3074

Merged

2opremio merged 3 commits intostellar:release-horizonclient-v4.0.0from 2opremio:release-horizonclient-update-changelogs

Sep 29, 2020

+30-16

Commits on Sep 29, 2020