Don't reset keepalive timer on received message

[droserasprout]

Resetting timer on received message leads to server closing connection due to inactivity. Protocol specs mention only sent messages as a reason to skip ping:

A Ping is only sent if the interval has elapsed without a message being sent.

[codecov]

Codecov Report

Merging #58 (37d835b) into main (13aaa64) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #58      +/-   ##
==========================================
- Coverage   94.11%   94.06%   -0.05%     
==========================================
  Files          34       34              
  Lines        1274     1264      -10     
==========================================
- Hits         1199     1189      -10     
  Misses         75       75              

Impacted Files	Coverage Δ
...lrcore/transport/websockets/websocket_transport.py	83.66% <ø> (-0.11%)	⬇️
signalrcore/protocol/base_hub_protocol.py	92.00% <0.00%> (-0.16%)	⬇️
signalrcore/helpers.py	100.00% <0.00%> (ø)

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[jdeksup]

Please merge this PR. We get a connection close from the serveer, just after the server send several events.