trickle: Various fixes

[j0sh]

• Fixes a bug that caused us to drop very small writes. Small miracle that we haven't been bitten by this yet.

• After closing a trickle channel, do a better job of cleaning up pre-connected publisher segments in a couple places. In practice this was mostly harmless (connections would have eventually errored out after enough 100 Continues) but this made unit tests take longer than they should.

• Add unit tests to verify all the above

We should also probably have more integration type tests to also exercise similar parts of the pytrickle clients but that can come later.

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trickle: Don't drop small reads.
We sometimes get bytes and an io.EOF from the same read call.

Errors are checked first but this would cause us to drop any data.

Check for the bytes first. The EOF can come on the next read call.

trickle: Clean up preconnected publishers.
There are a few parts to this fix, mostly on the server:

• Uses a channel to signal any pending connections

• Set Connection: close header to encourage client disconnects

• Force close the connection under some circumstances

• Specifically handle closed preconnects on the gotrickle client.

trickle: Add basic unit tests
Also run them in CI

[codecov]

Codecov Report

Attention: Patch coverage is 80.76923% with 20 lines in your changes missing coverage. Please review.

Project coverage is 31.71620%. Comparing base (ae88972) to head (75053c6).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
trickle/trickle_server.go	86.48649%	8 Missing and 2 partials ⚠️
trickle/trickle_subscriber.go	72.22222%	4 Missing and 1 partial ⚠️
trickle/local_subscriber.go	0.00000%	4 Missing ⚠️
trickle/trickle_publisher.go	87.50000%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3594         +/-   ##
===================================================
+ Coverage   30.54979%   31.71620%   +1.16641%     
===================================================
  Files            156         156                 
  Lines          47182       47244         +62     
===================================================
+ Hits           14414       14984        +570     
+ Misses         31926       31378        -548     
- Partials         842         882         +40     

Files with missing lines	Coverage Δ
trickle/trickle_publisher.go	62.98077% <87.50000%> (+62.98077%)	⬆️
trickle/local_subscriber.go	0.00000% <0.00000%> (ø)
trickle/trickle_subscriber.go	68.87417% <72.22222%> (+68.87417%)	⬆️
trickle/trickle_server.go	67.35967% <86.48649%> (+67.35967%)	⬆️

... and 3 files with indirect coverage changes

---

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Files with missing lines	Coverage Δ
trickle/trickle_publisher.go	62.98077% <87.50000%> (+62.98077%)	⬆️
trickle/local_subscriber.go	0.00000% <0.00000%> (ø)
trickle/trickle_subscriber.go	68.87417% <72.22222%> (+68.87417%)	⬆️
trickle/trickle_server.go	67.35967% <86.48649%> (+67.35967%)	⬆️

... and 3 files with indirect coverage changes

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[j0sh]

Added a couple more fixes and test cases - I'm going to merge this now before this PR gets even more unwieldy. Later fixes will have separate / smaller PRs