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
chore(engine): add comments regarding cmmn sentry features

related to CAM-2373
chore(engine): code cleanup

related to CAM-2839
chore(maven): cleanup pom.xml based on eclipse warnings

related to #CAM-2901
chore(mybatis): upgrade mybatis from 3.2.3 to 3.2.8

related to #CAM-2877
chore(build): add mvn profiles for servers and serverless

- add profile distro-tomcat
- add profile distro-gf31
- add profile distro-jbossas7
- add profile distro-serverless

related to #CAM-2876
feat(engine): make message name optional for correlation

- at least one of the following correlation criteria has to be supplied:
  - message name
  - business key
  - process instance id
  - correlation keys

related to CAM-1453
refactor(engine): deletion of historic details

related to CAM-2912
test(engine): deletion of transient cached historic details

- ensure that cached historic details that are staged to be inserted into the
  database are not in fact inserted when all historic details of the process
  instance are deleted in the same transaction

related to CAM-2912
chore(procDef/query): allow to query with name/nameLike and latest

related to CAM-2758
fix(enine): assignee set from CREATE listener missing in history

fix dirty checking for transient entities

related to #CAM-2921
chore(engine): remove duplicate code

related to #CAM-1453
chore(sql): use ${trueConstant} instead of 1

- replace 1 with ${trueConstant}
- remove redundant queries

related to #CAM-2900
updt(contribute.md): centralize all info on contributing here
wip
wip
tests