Adjust server tolerance to frequent keepalive pings

[sevein]

This avoids the following error seen in some clients while processing big transfers:

ipv4:127.0.0.1:42625: Received a GOAWAY with error code ENHANCE_YOUR_CALM and debug data equal to "too_many_pings". Current keepalive time (before throttling): ∞

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (7821c59) 45.85% compared to head (6254dd4) 45.85%.

Files	Patch %	Lines
a3m/server/runner.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #451   +/-   ##
=======================================
  Coverage   45.85%   45.85%           
=======================================
  Files         106      106           
  Lines        7413     7413           
  Branches     1202     1202           
=======================================
  Hits         3399     3399           
  Misses       3842     3842           
  Partials      172      172           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.