fix(engine): add connector as variable scope to scripts in IO mapping

- allows to use scripts as outputParameter of connectors
- make connector variable scope available as `connector` variable
- if the variable scope is not of type ExecutionEntity, TaskEntity
  or ConnectorVariableScope it will be available as `scope` variable

related to #CAM-2340
fix(engine): map null value variables inside call activity

related to #CAM-2401
chore(case/context): set case execution context on thread local

... and provide context switch to call custom code.

related to CAM-2239
LDAP group query encoding fix
fix: Escape id in LDAP user search filter
fix: Find users by group id assumed user DN always contained uid
Re-based from upstream
Merge branch 'master' of https://github.com/david2202/camunda-bpm-platform