Revert "HBASE-11234 FastDiffDeltaEncoder#getFirstKeyInBlock returns wrong result" due to test issues

This reverts commit b2a2e060982634cfae4bec2665f2b63e1f680ff1.
Revert "updated CHANGES.txt for 0.94.20RC0", remove a jira again

This reverts commit c8fb799b1010df9c340641acb2c90f752bba776a.