Management SSE for apps fails when Accept-Encoding is sent #8518

[rymsha]

Main fix: disabled gzip for SSE. At least for now. see #8518 (comment)

A few other polishes:

• For some reason Resteasy injects Context twice.
  https://github.com/resteasy/Resteasy/blob/master/resteasy-core/src/main/java/org/jboss/resteasy/core/ResourceMethodRegistry.java#L228
  https://github.com/resteasy/Resteasy/blob/master/resteasy-core/src/main/java/org/jboss/resteasy/core/ResourceMethodRegistry.java#L246
  TODO: Create an issue for Resteasy? https://issues.redhat.com/browse/RESTEASY-2779

That causes race conditions for us, so sse Context variable is set, I ignore others

• In case of not yet set contextHolder we should throw an error (well, it never happens, but still...), otherwise, if no error is thrown, client just hangs and never receives any messages

• Last fix is to send "list" message before any others can potentially arrive.

[codecov]

Codecov Report

Merging #8520 (6409dfe) into master (0f1d172) will decrease coverage by 0.00%.
The diff coverage is 83.33%.

@@             Coverage Diff              @@
##             master    #8520      +/-   ##
============================================
- Coverage     82.08%   82.08%   -0.01%     
  Complexity    19588    19588              
============================================
  Files          2769     2769              
  Lines         70464    70467       +3     
  Branches       5792     5793       +1     
============================================
+ Hits          57842    57844       +2     
- Misses         9457     9458       +1     
  Partials       3165     3165              

Impacted Files	Coverage Δ	Complexity Δ
.../com/enonic/xp/impl/server/rest/SseEntryPoint.java	93.22% <80.00%> (-1.52%)	14.00 <1.00> (ø)
.../web/jetty/impl/configurator/GZipConfigurator.java	100.00% <100.00%> (ø)	3.00 <0.00> (ø)
...ic/xp/core/impl/content/ParentProjectSyncTask.java	91.83% <0.00%> (ø)	12.00% <0.00%> (ø%)

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[rymsha]

issue about double injection submitted https://issues.redhat.com/browse/RESTEASY-2779