generator: perform a two-phase apply

[lucab]

Right now torcx-generator seals the system only at the end of a sucessful apply. This is used both to signal that a system has been properly set up, and to avoid performing propagation tasks again if the systemd gets daemon-reloaded.

This however doesn't work too well in the case where the generator fails for whatever reason and a later daemon-reload is performed. In this case, torcx will try to unpack addons again possibly on top of some already existing assets.

As this is a failure that we can't anyway solve at runtime, we should instead write a transaction-start entry to the FS to signal that torcx already run on this boot-instance (even if later failed).