immediately remove reset tokens when retiring a connection ID

[marten-seemann]

Otherwise, packet reordering could result in a stateless reset and an unwanted termination of the session.

It will probably be wise to cut another patch release after merging this PR.

[codecov]

Codecov Report

Merging #2897 (05066ae) into master (69158cf) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2897      +/-   ##
==========================================
+ Coverage   85.77%   85.79%   +0.01%     
==========================================
  Files         133      133              
  Lines        9187     9175      -12     
==========================================
- Hits         7880     7871       -9     
+ Misses        959      956       -3     
  Partials      348      348              

Impacted Files	Coverage Δ
internal/flowcontrol/base_flow_controller.go	100.00% <ø> (ø)
packet_handler_map.go	73.81% <ø> (+0.79%)	⬆️
session.go	77.27% <ø> (ø)
conn_id_manager.go	93.48% <100.00%> (-0.07%)	⬇️
internal/flowcontrol/connection_flow_controller.go	100.00% <100.00%> (ø)
internal/flowcontrol/stream_flow_controller.go	100.00% <100.00%> (ø)

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