feat(engine): enable async start events

- a start event may be declared async such that on execution, a new
process instance is created, but no activity executed yet

related to CAM-424
test(engine): test execution listener invocation on async start

related to CAM-424
fix(engine): fix execution listener invocation on async start event

related to CAM-424
improve(engine): support null configuration in async job handler

- supports migrated jobs that do not have a configuration value set

related to CAM-424
Merge branch 'CAM-424'

Conflicts:
	engine/src/main/java/org/camunda/bpm/engine/impl/persistence/entity/ExecutionEntity.java
Merge branch 'master' of github.com:camunda/camunda-bpm-platform