Automatically delete connections upon process completion/kill

[renatomassaro]

The Process signal system should automatically delete underlying connections - unless declared otherwise.

Note that this behaviour is desired when the process is deleted, regardless of the signal (SIGTERM or SIGKILL, for instance), so it should create a handler that also lets the Processable know why the process was deleted. Another implementation option is to extend the usual {:delete, [Event.t]} with signatures that include, for instance, [delete_src_connection: false].

Example scenario: FileTransferProcess creates FTP connection that should automatically be deleted upon completion/abortion.

Look for references to #388