DefaultAclManager.reset doesn't reset groups/users/mappings

hi kpauls while reviewing changes for SLING-10920 i noticed that the sets capturing users/groups as well as service-user-mappings are not reset in the reset() method.

i am not entirely sure if the mappings if the mappings should be reset as they might present some other type of global information that needs to be remembered across different packages.... maybe the same applies for users/groups.....
if it shouldn't be reset, we should add a comment explaining it.

wdyt?