Avoid duplicate tree resolution in MembershipProvider

as a follow-up to OAK-9737 we should also look into the MembershipProvider which resolves references to members and for membership calculation. as described in OAK-9737 the underlying query already resolves the tree objects but the code retrieves the path of groups/members and subsequently obtains the tree objects a second time.

i decided to split this improvement from OAK-9737 as it requires a bit more refactoring (not just limited to a the inner workings of individual methods).

cc: thomasm, joerghoh