Fixed various subscribe issues

[samniisan]

• Fixed the multiple subscription issue (when subscribing multiple times with the same filters, Socket.io wouldn't use the already existing connexion and created a new one, the proxy would then consider that the subscription was made by anoter client)
• Crash when receiving a Notification with "unknown" scope (delete queries), missing value in ENUM
• Fixed missing _meta in Document entity created upon receiving a Document Notification (create, replace etc.)

[codecov-io]

Codecov Report

Merging #147 into 3.x will decrease coverage by 0.07%. The diff coverage is 33.33%.

@@            Coverage Diff             @@
##              3.x     #147      +/-   ##
==========================================
- Coverage   91.49%   91.42%   -0.08%     
==========================================
  Files          32       32              
  Lines        4692     4697       +5     
  Branches      438      439       +1     
==========================================
+ Hits         4293     4294       +1     
- Misses        272      276       +4     
  Partials      127      127

Impacted Files	Coverage Δ
src/main/java/io/kuzzle/sdk/enums/Users.java	100% <ø> (ø)	:arrow_up:
.../io/kuzzle/sdk/responses/NotificationResponse.java	28.57% <0%> (-3.69%)	:arrow_down:
src/main/java/io/kuzzle/sdk/core/Kuzzle.java	92.24% <100%> (ø)	:arrow_up:
src/main/java/io/kuzzle/sdk/enums/Scope.java	100% <100%> (ø)	:arrow_up:

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