Add a max capacity and old entries eviction

[sandhose]

• Implement maximum capacity and eviction
• Add a flag to fill the entire map, to check how much memory it uses

[codecov-commenter]

Codecov Report

Base: 82.71% // Head: 77.27% // Decreases project coverage by -5.43% ⚠️

Coverage data is based on head (370ae9f) compared to base (89eabe2).
Patch coverage: 64.77% of modified lines in pull request are covered.

❗ Current head 370ae9f differs from pull request most recent head 6bbab81. Consider uploading reports for the commit 6bbab81 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #5      +/-   ##
==========================================
- Coverage   82.71%   77.27%   -5.44%     
==========================================
  Files           3        3              
  Lines         509      660     +151     
==========================================
+ Hits          421      510      +89     
- Misses         88      150      +62     

Flag	Coverage Δ
unit	77.27% <64.77%> (-5.44%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/src/main.rs	1.66% <0.00%> (-0.90%)	⬇️
synapse/src/lib.rs	2.27% <0.00%> (-0.36%)	⬇️
src/lib.rs	91.36% <77.02%> (-5.63%)	⬇️

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