Use new timestamp helper

Remaining `utcfromtimestamp` call sites:

```
mimic/session.py
91:                datetime.utcfromtimestamp(self.clock.seconds())
154:            expires=datetime.utcfromtimestamp(self.clock.seconds() +

mimic/test/test_session.py
29:                         datetime.utcfromtimestamp(4321 + 86400))
85:            a.expires, datetime.utcfromtimestamp(A_LITTLE + A_LOT))

mimic/util/helper.py
42:    return datetime.utcfromtimestamp(seconds).strftime(format)
97:    current_datetime = datetime.utcfromtimestamp(current_timestamp)
```

All of these either implement the helper, or really want a `datetime`, not a formatted timestamp.

Closes #115.