NMS-13132: derive aggregations for hosts and applications from the aggregation for conversations

[swachter]

Issue: https://issues.opennms.org/browse/NMS-13132

• Calculates aggregations for hosts/applications on-top of the aggregations for conversations
• The Aggregation class now can track two host names (in case of conversation it tracks the src and dst hostname)
• Key creation logic is simplified by decoupling it from src/dst hostname logic. Now, only a single key is returned for a document (previously key creation could return two WithHostname instances capturing the src and dst hostnames). Src/dst hostnames are now considered when the keys of conversation aggregations are mapped into two host keys for the two hosts taking part in that conversation.