Use LruHashMap over HashMap for pending messages + Other Fixes

[tbraun96]

This PR is an attempt to move us closer to memory stability. As bugs are found they will be fixed here.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: +0.01 🎉

Comparison is base (ca02f61) 16.10% compared to head (6119e68) 16.10%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #673      +/-   ##
==========================================
+ Coverage   16.10%   16.10%   +0.01%     
==========================================
  Files          73       73              
  Lines        5413     5412       -1     
==========================================
  Hits          871      871              
+ Misses       4542     4541       -1     

Impacted Files	Coverage Δ
dkg-gadget/src/gossip_engine/network.rs	0.00% <0.00%> (ø)
...-gadget/src/gossip_messages/misbehaviour_report.rs	0.00% <0.00%> (ø)
dkg-test-orchestrator/src/dummy_api.rs	0.00% <ø> (ø)

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