Refactor commands and implement event handlers.

Fixes #3877
Fixes #4001
Implement command timeouts.

Fixes #2723
Implement volatile services

Fixes #3593
Removed obsolete file.
Update tutorial.
Remove unused typedef.
Don't seal DynamicObject updates.
Implement support for downtime comments.

Fixes #3894
Implement support for acknowledgement comments.

Fixes #3585
Bugfix for removing expired downtimes.
Add missing lock.
Make error message more user-friendly.
Config: Fix nested dictionaries in arrays.
Implement flapping detection.
Implement external commands for flapping detection.
Update the flapping detecting formula.
docs: s/check_command/command/ on command object, s/CGIs/Classic UI/

"CGIs" is an old definition, it's now referenced as "Classic UI" within
the icinga eco system.
Fix: Compat Log: abort on OwnLock()

Fixes #4332
Remove obsolete templates.
Fix: missing check_command attribute results in sigsegv on check execution

Fixes #4331