Support spontaneous reboots during artifact script execution

Acceptance criteria

• All artifact script execution and results must be recorded in persistent storage, so that a reboot will pick up where it left off
• All states and transition context must be recorded in persistent storage, so that a reboot will pick up where it left off.
• Upon starting, Mender must check which state it was in and rerun the state if it was not finished
• the only guarantee given by Mender is that the state or transition that was currently in progress when the spontaneous reboot happened, will be rerun.
  -> Thus the user will have to guarantee that all the functionality required in one state/transition is capable of operating independently of the other states/transitions.
• Time that the script was first executed must also be recorded so that timeout may not be artificially lengthened by rebooting