Find a way to clear ncdns cache state on new NYM_EPOCH

[JeremyRand]

Currently, ncdns's cache doesn't delete old stream isolation ID's, which means the cache will grow without bound as Tor Browser creates more circuits. We should look into something along these lines:

1. StemNS detects that NYM_EPOCH has changed (it does this anyway to clear the StemNS stream isolation cache).
2. StemNS sends a NEWNYM command over Prop279 to ncprop279.
3. ncprop279 instructs the ncdns backend to delete all cache entries.