Recursive memberOf group search strategy

This is a companion to the recursive member search strategy introduced in #64 which searches for the groups an entry is a member of.

There isn't really a *classic* strategy here other than to just search for the direct memberships without recursing to find subgroups the entry is a member of.

Going to follow up after this is merged to clean up and bring this and #64 functionality into a more consistent interface.

cc @jch @github/ldap 

This looks pretty innocuous, but it can result in N*M *recursive* queries and exceptionally bad performance for large data sets.

It might make more sense to inline the relevant logic from the *search strategy* in order to recursively search for multiple entries at a given time (instead of individually). This warrants more consideration and testing.