Change to history plugin's way of getting time in previous state that accounts for state changes at H1 at NLN

Resolves #210 (closed) This update is to the history plugin so when it is running the look-back to see how long we were in the state before the lockloss, if we are at H1 and in state 600 or above, we take into account that entire time that we were in nominal low noise when returning the 'previous state' gps time. Previously, the look-back would only run back in time from the lockloss until it finds when the last state transition took place, but this means that if we went to NLN, NLN_CAL_MEAS, and then back to NLN before losing lock, the 'time in state' would only show the time from the last transition, even though we had been in the low noise state since arriving to NLN the first time. This change now accounts for these situations and aids in getting a better view of the actual time in low noise.