[Job executor] Switch to use executor Service interface vs ThreadPoolExecutor (nicer for pluggability)
Merge branch 'job-executor-work' of https://github.com/Activiti/Activiti into job-executor-work

Conflicts:
	modules/activiti-engine/src/main/java/org/activiti/engine/impl/asyncexecutor/DefaultAsyncJobExecutor.java
	modules/activiti-engine/src/main/java/org/activiti/engine/impl/cfg/ProcessEngineConfigurationImpl.java