Fix wantlist overflow handling to select newer entries.

[gammazero]

bitswap wantlist overflow handling now selects newer entries when truncating wantlist. This fix prevents the retained portion of the wantlist from filling up with CIDs that the server does not have.

Fixes #527

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.83%. Comparing base (dfd4a53) to head (0126d1f).

@@            Coverage Diff             @@
##             main     #627      +/-   ##
==========================================
+ Coverage   59.79%   59.83%   +0.03%     
==========================================
  Files         238      238              
  Lines       29984    29983       -1     
==========================================
+ Hits        17930    17939       +9     
+ Misses      10434    10427       -7     
+ Partials     1620     1617       -3     

Files	Coverage Δ
bitswap/server/internal/decision/engine.go	91.75% <100.00%> (+0.53%)	⬆️

... and 12 files with indirect coverage changes

[gammazero]

Need different approach. Old entries need to be canceled.