dns: refactor update() in NMDnsSystemdResolved to use a hash table
Use a GHashTable instead of a GArray to construct the list of @interfaces. Also, use NMCListElem instead of GList. With this, the runtime is O(n*log(n)) instead of O(n^2). I belive, we should take care that all our code has a reasonable runtime complexity, even in common use-cases the number of elements is small. This is not about performace, because likely we expect few entries anyway, and the direct GArray implementation is likely faster in those cases. It's about using the data structure that best suits the access pattern. The log(n) part comes from sorting the keys. I also believe we should always aim for a stable behavior. When sending the D-Bus request to resolved, the order of elements should be in ~some~ defined order.
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