Fix "Futures re-subscribe to authenticated feed doesn't work"

[btschwertfeger]

Summary

This MR fixes the mentioned bug using the suggested change - as described in #179 by setting the __challenge_ready-flag to False in case of a failed task to enforce setting a new challenge and not using the old/broken one.

More changes and additions:

• Add tests (also for the Spot websocket clients) to check that reconnect and resubscribe behaviour after a temporary connection lost
• Adjust the internal caching of subscriptions for Spot to prevent failing resubscriptions due to Krakens current subscription-response behaviour for WS V2.

[codecov]

Codecov Report

Merging #180 (52dd055) into master (2e430bb) will increase coverage by 1.93%.
The diff coverage is 95.83%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #180      +/-   ##
==========================================
+ Coverage   87.79%   89.73%   +1.93%     
==========================================
  Files          21       21              
  Lines        1877     1890      +13     
==========================================
+ Hits         1648     1696      +48     
+ Misses        229      194      -35     

Flag	Coverage Δ
unittests	89.73% <95.83%> (+1.93%)	⬆️

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Files	Coverage Δ
kraken/futures/websocket/__init__.py	88.27% <100.00%> (+9.65%)	⬆️
kraken/spot/websocket/connectors.py	91.26% <94.44%> (+10.64%)	⬆️