Refactor UnLock -> ConflictDetector

[njsmith]

Just rename it and make it always do ResourceBusyError, since that's
how it's always used.

Fixes gh-197

[codecov]

Codecov Report

Merging #295 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #295      +/-   ##
==========================================
- Coverage   99.18%   99.18%   -0.01%     
==========================================
  Files          79       79              
  Lines        9665     9660       -5     
  Branches      686      686              
==========================================
- Hits         9586     9581       -5     
  Misses         62       62              
  Partials       17       17

Impacted Files	Coverage Δ
trio/_ssl.py	100% <100%> (ø)	⬆️
trio/testing/_memory_streams.py	99.56% <100%> (ø)	⬆️
trio/_util.py	92.55% <100%> (-0.08%)	⬇️
trio/_highlevel_socket.py	100% <100%> (ø)	⬆️
trio/tests/test_ssl.py	100% <100%> (ø)	⬆️
trio/tests/test_util.py	100% <100%> (ø)	⬆️

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