utilize ServiceInfo from database for collect-startup and serve-tasks

[wagnerd3]

Checklist:

• If this PR is about a plugin, I tested the plugin against an OpenStack cluster.
• I updated the documentation to describe the semantical or interface changes I introduced.

Contents of this PR:

• collect task falls back to database values in case of Liquid not being available on startup
• non-collect tasks always use values from the database without cache, through the Cluster object
  • the DataMetricsReporter is the only user which caches the info for the duration of one assembly of the metrics
• only collector-stuff accesses the LiquidConnections directly

This is not yet tested properly for all cases, because the datasources in qa-de-1 are partially down. Therefore opening as draft for now.

[coveralls]

coverage: 78.295% (-0.8%) from 79.123%
when pulling e6a7e7c on utilize_service_info_from_db2
into fcd2bfb on master.

[wagnerd3]

I have adressed the comments from @majewsky now. I initially started to pass through the errors of all the small helper functions everywhere, but quickly realized that by checking for the errors everywhere the code gets very messy. Additionally, because of the many specific methods to access the ServiceInfo from the DB, it was hard to keep track of double-access to the DB.

I did solve this a little bit different now: On Cluster level, the access is just possible to access all ServiceInfos as a whole - for all services or just for a single one. The utility functions get this object. This way, it is much easier to track where the object should be assembled. Sometimes, this means we do 1 unnecessary DB call for the rates, but this is still better than doing X calls for every single hasService etc.

I had to add this serviceInfos object to very many function signatures to pass it through, so I am not yet sure whether this is the final/ nicest solution. Better ideas are always welcome.

[majewsky]

Note to myself, I have not checked internal/collector/metrics.go yet because I ran out of time on this review.