Remove peer's count of first responses when peer becomes unavailable

[gammazero]

The peer response tracker counts the times each peer is first to response to block requests. When a peer becomes unavailabile, its count of first responses should be removed. This way we are not tracking the first response count for peers that are no longer available.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.47%. Comparing base (8854a6f) to head (28cd43f).
Report is 1 commits behind head on main.

@@            Coverage Diff             @@
##             main     #757      +/-   ##
==========================================
+ Coverage   60.37%   60.47%   +0.10%     
==========================================
  Files         245      245              
  Lines       31114    31117       +3     
==========================================
+ Hits        18784    18818      +34     
+ Misses      10654    10633      -21     
+ Partials     1676     1666      -10     

Files with missing lines	Coverage Δ
...wap/client/internal/session/peerresponsetracker.go	96.42% <100.00%> (+0.27%)	⬆️
...tswap/client/internal/session/sessionwantsender.go	97.65% <100.00%> (+1.41%)	⬆️

... and 15 files with indirect coverage changes