Serialise terminated instances

In order to retrieve information post-mortem (done, failed, lost) for an instance we need to store it as the original data is lost with the removal of the instance directory. This change-set serialises the instance state into JSON as the value for the already existing keys in the procs subpath. Together with that it implements a method to retrieve the state and turns it into an `Instance` again.

This should be used in tooling (cli) to retrieve the list of all done instances.
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/cc @bernerdschaefer @grobie @peterbourgon