Add Cancellable interface.

  - Provides a tiny subset of RunnableFuture.
Add mvn -DEBUG system property support to ILC.

  - Just run mvn with the "-DEBUG" option to automatically enable
  debug logging for the StatusLogger when using ILC.
Update configuration manual for ShutdownCallbackRegistry rename.
Create customizable ShutdownCallbackRegistry.

  - Implements LOG4J2-868.
  - Renamed ShutdownRegistrationStrategy to ShutdownCallbackRegistry.
  - Shutdown callback registry has been scoped to Log4jContextFactory
  for use in application containers.
  - ShutdownCallbackRegistry can optionally implement LifeCycle.
  - DefaultShutdownCallbackRegistry can also be extended.
  - Added constructors to Log4jContextFactory (mainly seems useful
  for subclasses).
Add test for ShutdownCallbackRegistry.
Update changelog for LOG4J2-868.