Support checking for and re-collecting updated data

[chad-earthscope]

As of 0.0.6, updated data is not checked for.

Rover could use showlastupdate=true in the call to the availability service, which will then result in a most-recent-update update column to be included in the response. This date-time value should be compared to the updated date-time value in the local index to determine whether the local copy is the latest. These data should then be replaced.

How to replace data. Data could be replaced at the station-day granularity (potentially re-downloading channels that have not been updated); basically re-fetching all data in for the given station-day according to the subscription/request, replacing the station-day file and re-indexing it.

Another option, have rover carve out the data that needs to be replaced based on the byte ranges in the index (no miniSEED parsing needed) and re-refetching just the updated channels that are then appended to the station-day file per a normal retrieve. This cold leverage the "data removal" functionality in #21.