Refactor the process_station_measures to work on a per station key basis.

This lets us remove a bunch of the dictionaries and state tracking and
make sure we only check the blacklist and free space once per station.
Use the space available check to also determine new station status.

This avoids one extra query to the station table. Also avoid blacklist
check for already known stations.
Merge pull request #264 from hannosch/process-station-by-key

Refactor the process_station_measures to work on a per station key basis.