Fix virtual group search.

There are a few things wrong with this implementation. I've noticed that members of a group are not returned even when the overlay is working properly.

```
ubuntu@ubuntu:~$ sudo ldapsearch -LLL -Y EXTERNAL -H ldapi:/// -b "dc=github,dc=com" "(memberof=cn=Assets,ou=teams,dc=github,dc=com)"
SASL/EXTERNAL authentication started
SASL username: gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth
SASL SSF: 0
dn: uid=calavera,ou=users,dc=github,dc=com
cn: calavera
sn: calavera
uid: calavera
mail: calavera@github.com
objectClass: inetOrgPerson

dn: uid=pengwynn,ou=users,dc=github,dc=com
cn: pengwynn
sn: pengwynn
uid: pengwynn
mail: pengwynn@github.com
objectClass: inetOrgPerson

dn: uid=rubyist,ou=users,dc=github,dc=com
cn: rubyist
sn: rubyist
uid: rubyist
mail: rubyist@github.com
objectClass: inetOrgPerson

dn: uid=sbryant,ou=users,dc=github,dc=com
cn: sbryant
sn: sbryant
uid: sbryant
mail: sbryant@github.com
objectClass: inetOrgPerson

dn: uid=technoweenie,ou=users,dc=github,dc=com
cn: technoweenie
sn: technoweenie
uid: technoweenie
mail: technoweenie@github.com
objectClass: inetOrgPerson
```

:warning: **WIP** :warning:

/cc @mtodd