routing_table: Merge overlapping contacts during configuration

Previously we would simply ignore any contact that was overlapping. This changes the routing table's behavior such that overlapping contacts with the same node are merged and overlapping contacts with multiple different nodes are just allowed and added alongside each other. The maximum number of contacts defined by MAX_CONCURRENT_CONTACTS now is just a limitation of the Contact Manager which will not process more than that number of contacts at the same time. (If one of them ends, a concurrent contact that was not handled can be added.)

This fixes an issue where we could not add new reachable EIDs to any contact that had been scheduled previously, as the newly-added contact was considered an "invalid overlap".

Fixes: #99 (closed)

Based upon !80 (merged)