Properly handle tokenExpired events

[scottinet]

Description

The Kuzzle event tokenExpired should be triggered in two cases:

• when a request response contains tokenExpired error
• whenever a TokenExpired realtime event is received

The current SDK version only handles the first case, which causes many problems for clients using realtime, such as:

• realtime callbacks are not cleared, meaning clients handling manually the event have more and more of the same callbacks registered (and invoked)
• the tokenExpired event is not triggered upon receiving a TokenExpired realtime notification
• the JWT is not cleared upon receiving a TokenExpired realtime notification

This PR adds a proper TokenExpired realtime notification handling, ensuring that all the above problems are solved.

[codecov-io]

Codecov Report

Merging #374 into 6-dev will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##            6-dev     #374      +/-   ##
==========================================
+ Coverage   96.61%   96.64%   +0.03%     
==========================================
  Files          28       28              
  Lines        1417     1430      +13     
==========================================
+ Hits         1369     1382      +13     
  Misses         48       48

Impacted Files	Coverage Δ
src/controllers/realtime/room.js	100% <100%> (ø)	⬆️
src/controllers/realtime/index.js	100% <100%> (ø)	⬆️

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