Move to a new model for data stored in redis

- Keep sets to index instances/apps/roles so it's not necessary
 to scan the keyspace using keys command all the time
- Added datahelper module and moved almost all call to redis there,
  so keys can be changed or prefixed in future without hunting for them
  everywhere
- Better tracking of unregistered servers
- Simplified rehash
- Add migrate script to convert data from old version (useful on upgrade
  and in case we want to restore an old dump)

Bumped major version to 0.6.0