GerritChangeSource: avoid logging of unecessary messages.

[wainersm]

Currently GerritChangeSource log a message for every
received event that doesn't have a type or which
isn't set to be handled. This pollute the log file
with unecessary messages.

This change enable logging of these messages only when
debug is set True.

[mention-bot]

@wainersm, thanks for your PR! By analyzing the history of the files in this pull request, we identified @tardyp, @sa2ajj and @jvlomax to be potential reviewers.

[codecov]

Codecov Report

Merging #3036 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3036      +/-   ##
==========================================
- Coverage    87.5%   87.49%   -0.01%     
==========================================
  Files         313      313              
  Lines       33092    33199     +107     
==========================================
+ Hits        28956    29047      +91     
- Misses       4136     4152      +16

Impacted Files	Coverage Δ
master/buildbot/changes/gerritchangesource.py	74.64% <ø> (+0.46%)	✅
master/buildbot/process/factory.py	64.4% <0%> (-1.25%)	❌
master/buildbot/process/remotetransfer.py	95.08% <0%> (+3.22%)	✅
worker/buildbot_worker/commands/base.py	36.64% <0%> (+5.92%)	✅

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