feat(engine): add cmmn variable listeners

- similar to execution listeners, allows to define variable listeners
  on model scopes that are notified whenever a variable changes in
  this scope or any child scopes
- adds configuration flag to the engine configuration to disable custom
  listener invocation

related to CAM-2238
chore(engine): code cleanup

related to CAM-2238
fix(engine): servlet PA deployer on WAS

improve WAS 8.5 workaround

related to #CAM-2881
test(engine): add item handler test for variable listeners

related to CAM-2238