Move heartbeat timer into session state

[davidsteiner]

This ensures that heartbeats only tick when we're in a valid state for heartbeating.

[codecov]

Codecov Report

❌ Patch coverage is 78.26087% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.42%. Comparing base (a3b1d61) to head (40a5f19).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/hotfix/src/session.rs	57.14%	3 Missing ⚠️
crates/hotfix/src/session/state.rs	87.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #133      +/-   ##
==========================================
+ Coverage   62.27%   62.42%   +0.15%     
==========================================
  Files          66       66              
  Lines        2749     2760      +11     
==========================================
+ Hits         1712     1723      +11     
  Misses       1037     1037              

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