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`persistence` represents the data that does not get lost between server crashes and restarts. We store a log of commands in `_stored_entries`. It is invariably ``contiguous'', meaning that the index of each entry except the first is equal to the index of the previous entry plus one at all times (i.e. after each yield). We assume that the caller provides log entries in strictly increasing index order and without gaps. Additionally to storing log entries, `persistence` can be asked to store or load a snapshot. To implement this it takes a reference to a set of snapshots (`snapshots_t&`) which it will share with `impure_state_machine` and an implementation of `rpc` coming in a later commit. We ensure that the stored log either ``touches'' the stored snapshot on the right side or intersects it.