Read status/values of parameters in backend

[SimonHeybrock]

Basically we need to implement a gather of information from all ranks (via BackendHeartbeat). The root rank is then exposing this via the ParameterControlServer.

We need this in particular for monitoring queue status and memory consumption.

[SimonHeybrock]

The way this should be implemented basically depends on whether or not we can accept (1) latency and (2) blocking behavior.

• If blocking is acceptable, the setter that is called by ParameterControlServer can request the info from all ranks via the heartbeat and wait for the reply.
• If blocking is not acceptable but latency is acceptable, we can return immediately from the setter. The result must then be delivered later via other means.
• If neither blocking nor latency is acceptable, we can continuously gather status information from all ranks on the root rank.

The latter option might actually be one of the easier ones. The getter would be trivial, we only need to implement the gathering. But how to define what is to be included in the status gather?