avoid duplicating ldaptor code in LDAPServerWithUPNBind

[graingert]

Contributor Checklist:

• [ ] I have updated the release notes at docs/source/NEWS.rst
• [ ] I have updated the automated tests.
• [ ] All tests pass on your local dev environment. See CONTRIBUTING.rst.

[codecov[bot]]

Codecov Report

Merging #161 into master will decrease coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #161      +/-   ##
==========================================
- Coverage   93.35%   93.35%   -0.01%     
==========================================
  Files          72       72              
  Lines        9915     9914       -1     
  Branches      973      972       -1     
==========================================
- Hits         9256     9255       -1     
  Misses        537      537              
  Partials      122      122

Impacted Files	Coverage Δ
ldaptor/protocols/ldap/ldapserver.py	80.45% <100%> (-0.07%)	:arrow_down:
ldaptor/test/test_server.py	100% <100%> (ø)	:arrow_up:

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[adiroiban]

Thanks for the cleanup. @graingert let us know when you think this is ready for review :)

[graingert]

@adiroiban I think it's ready to go