Allow to force kill agents

When killing an agent, the agent can be stuck waiting for a process and not be unregistered.

Allowing to force kill an agent would allow to forget the agent, even if it is seen as busy.

Example possible implementation

Add a --force flag on kill command. When present:

• first try to properly kill the agent and wait for termination to forget it
• if the agent stays unresponsive for a long period, forget it

TODO target version to be defined