startup: preserve the database

[hannesm]

There can be arbitrary complexity / stuff broken when starting albatross daemon:

• bridges may be missing (or being disabled)
• tap interfaces are not set to up_on_open
• whatever else

This leads to albatross starting, but all unikernels failing to start --> sigabort or some other issue with solo5-hvt dumping core. A fix of the system is good, but won't bring us back the unikernels <--> manual intervention needed.

It would be great to first copy the "albatross.state" to a "albatross.state.started" backup file, so we can recover (manually) from such failures, without needing to deal with each unikernel individually.