various amplification limit fixes

[marten-seemann]

Fixes #3053.

1. When becoming amplification limited, we don't need to set a loss recovery timer. All we can do at that point is wait for a packet from the client to arrive.
2. Once that packet arrives, we need to restart the loss recovery timer.

[codecov]

Codecov Report

Merging #3132 (b6634fe) into master (3376a9f) will decrease coverage by 0.01%.
The diff coverage is 78.95%.

@@            Coverage Diff             @@
##           master    #3132      +/-   ##
==========================================
- Coverage   85.48%   85.47%   -0.01%     
==========================================
  Files         131      131              
  Lines        9650     9663      +13     
==========================================
+ Hits         8249     8259      +10     
- Misses       1033     1035       +2     
- Partials      368      369       +1     

Impacted Files	Coverage Δ
internal/ackhandler/sent_packet_handler.go	77.91% <77.78%> (+0.22%)	⬆️
session.go	77.10% <100.00%> (ø)
internal/utils/rand.go	62.50% <0.00%> (-12.50%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3376a9f...b6634fe. Read the comment docs.