rec: RPZ updates are done zone by zone, zones are now shared pointers

rec: RPZ updates are done zone by zone, zones are now shared pointers

This prevents having to copy and update all the zones even though
the RPZ IXFR tracker only works on one of them at a time.
Also prevents race conditions if two RPZ IXFR tracker threads
update the state at the same time by using `GlobalStateHolder::modify()`
instead of `GlobalStateHolder::setState()`.