Check usage of global variables

[bernd]

Problem description

We are using global registries in some places which are not thread-safe and we have to check if this can become a problem.

I found global registries here:

• system/status.go: GlobalStatus = &Status{}
• assignments/assignment.go: Store = &assignmentStore{make(map[string]string)}
• backends/registry.go: Store = &backendStore{backends: make(map[string]*Backend)}

We should check if there are more.