Stop setting global subscription interval

Problem: Right now all connections are subscribed with the same subscription interval. This makes it redundant as we can just use the write interval from the Subhandler for this.

Eta-utility supports specifying intervals for Nodes. We can use these to determine the subscription interval for the connection. Each connection only supports one interval for each Node so we have to choose a interval from the given values.

Implementation Idea: Choose the minimum of all Node interevals or the default subscription interval if it is higher (which is passed as an argument)