feat: support long polling requests and event-source responses

[ronag]

[ronag]

Needs tests

[ronag]

This logic is only relevant when pipelining > 1 and ensures that no more requests are enqueued.

[codecov-io]

Codecov Report

Merging #538 (020f6ea) into master (8aa5ad1) will decrease coverage by 0.07%. The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master     #538      +/-   ##
==========================================
- Coverage   99.63%   99.56%   -0.08%     
==========================================
  Files          16       16              
  Lines        1365     1375      +10     
==========================================
+ Hits         1360     1369       +9     
- Misses          5        6       +1     

Impacted Files	Coverage Δ
lib/core/symbols.js	100.00% <ø> (ø)
lib/core/client.js	99.30% <92.85%> (-0.17%)	:arrow_down:
lib/core/request.js	100.00% <100.00%> (ø)

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[metcoder95]

This is an implementation of SSR or pure long-polling? (I'm asking because I see the text/event-stream header involved) :)