JAMES-4042 Make GC 'async safe'

[chibenwa]

• [ ] Refactoring: make runnign mode a parameter of the Task interface
• [ ] SerialTaskManagerWorkerTest: more test for async tasks (cancel, await, etc...)
• [ ] And finally add a async=true|false onto the GC webadmin endpoint.

[chibenwa]

Alternative to https://github.com/linagora/james-project/issues/5202