Procedure V2 replaced table DDL handler implementation to procedure implementation. It does not make sense to keep the *Handler methods in MasterObserver. This JIRA follows
HBASE-13645 way to deprecate the existing observers with *Handler and add the new observers with more meaningful names.
One proposal that to rename the coprocessor in procedure thread with Action and use Completed/Rollback to distinguish successful or failed procedures. (Note: this JIRA is just for name change; the actual Rollback cp does not exist today)