Remove Trillian Fakes

[gdbelvin]

Fakes are not-great, and maintaining them as the Trillian API changes is not sustainable.
Out with fakes -- in with mocks! We get fewer dependencies and more precise test assertions too!

Depends on google/trillian#1006

[codecov-io]

Codecov Report

Merging #928 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #928   +/-   ##
======================================
  Coverage    51.1%   51.1%           
======================================
  Files          34      34           
  Lines        2489    2489           
======================================
  Hits         1272    1272           
  Misses        988     988           
  Partials      229     229

Impacted Files	Coverage Δ
core/keyserver/keyserver.go	33.33% <100%> (ø)	⬆️

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