V6: Option "subscribeToSelf" using volatile data

[scottinet]

V6.x version of #268
fix #275

[codecov-io]

Codecov Report

Merging #269 into 6.x will increase coverage by 84.78%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##              6.x     #269       +/-   ##
===========================================
+ Coverage   13.74%   98.52%   +84.78%     
===========================================
  Files          17       17               
  Lines        2045     2029       -16     
  Branches      595      593        -2     
===========================================
+ Hits          281     1999     +1718     
+ Misses       1764       30     -1734

Impacted Files	Coverage Δ
src/Kuzzle.js	97.84% <100%> (+89.87%)	⬆️
src/networkWrapper/protocols/abstract/realtime.js	98.47% <100%> (+85.54%)	⬆️
src/SearchResult.js	97.72% <0%> (+29.54%)	⬆️
src/networkWrapper/index.js	100% <0%> (+53.84%)	⬆️
src/eventEmitter/index.js	97.14% <0%> (+72.85%)	⬆️
src/Document.js	98.97% <0%> (+73.46%)	⬆️
src/security/Role.js	100% <0%> (+73.68%)	⬆️
src/MemoryStorage.js	99.04% <0%> (+80%)	⬆️
src/security/Profile.js	100% <0%> (+80.43%)	⬆️
... and 9 more

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