Rec: Store the RPZ policies in an unordered_map instead of a map

Rec: Store the RPZ policies in an unordered_map instead of a map

This speeds up the loading of huge zones a bit (~20%) while also nicely
improving the lookup speed.
Also adds a `zoneSizeHint` parameter to `rpzFile()` and `rpzMaster()` to be
able to reserve space before loading the zone, to prevent reallocation
and rehashing when possible.