Rewrite SimpleDnsCache to be immutable

[rogeralsing]

For context, see the conversation here: #1222

The SimpleDnsCache is mutable, but shared globally in a system extension.
Thus, its open for race conditions.
I've labeled this confirmed bug, which might be a bit harsh as there is currently only one consumer, an actor of this class, so its not a visible bug atm.
but if anyone take dependency on the extension exposing this, it will be visble.

the scala version is based on immutability and we should follow their lead on this.