Use defaultdicts for BaseHubConnection handlers instead of lists

[sam-mosleh]

This PR uses defaultdicts for handlers in BaseHubConnection. Using lists can cause major performance issues.

[codecov]

Codecov Report

Merging #50 (44c5c04) into master (91a94ee) will increase coverage by 0.57%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   94.45%   95.03%   +0.57%     
==========================================
  Files          32       32              
  Lines        1227     1228       +1     
==========================================
+ Hits         1159     1167       +8     
+ Misses         68       61       -7     

Impacted Files	Coverage Δ
signalrcore/hub/base_hub_connection.py	82.58% <84.61%> (+1.58%)	⬆️
signalrcore/protocol/base_hub_protocol.py	91.66% <0.00%> (+4.16%)	⬆️
signalrcore/messages/close_message.py	100.00% <0.00%> (+40.00%)	⬆️

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