the stats module keeps track of frequent iterative queries using
probabilistic sampling. the list includes subrequests.
it is useful for tracking over time, prefetching and anomaly reporting

enumerated metrics are always present (set to 0 if not used), this fixes
missing values when not measuring and provides a faster access to common
data

by default it blocks queries to private zones, can be extended to match anything in the query content (i.e. water torture style attacks)

before root hints were hardcoded to the resolver,
now they are present in form of a cut in the resolution
context, and the modules can add/remove/replace them
on the fly

the Redis backend supports UNIX, TCP and custom ports
no real transactions nor iteration, but it’s not needed at the moment
transactions keep replies in a freelist, which is purged on transaction
commit / abort