feat(cmmn): add new db table ACT_RU_CASE_EXECUTION

This commit extends the db with a new table: ACT_RU_CASE_EXECUTION.
Furthermore the upgrade scripts are updated too.

related to CAM-2188
chore(cmmn): reorganize PlanItem to CaseExecution

This commit

- renames PlanItem to CaseExecution
- reorganize package structure (the Cmmn Impl stuff moved to the
  *.impl.cmmn.* packages)

related to CAM-2188