Decouple is_fresh from code_contractst

[tautschnig]

It really just needs a goto model, and otherwise has no dependency on code contracts. Also consistently use message handlers, not messaget.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Patch coverage: 96.00% and project coverage change: -0.01 ⚠️

Comparison is base (bce472f) 78.51% compared to head (9777e00) 78.51%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7679      +/-   ##
===========================================
- Coverage    78.51%   78.51%   -0.01%     
===========================================
  Files         1674     1674              
  Lines       191946   191943       -3     
===========================================
- Hits        150715   150711       -4     
- Misses       41231    41232       +1     

Impacted Files	Coverage Δ
src/goto-instrument/contracts/contracts.h	100.00% <ø> (ø)
...rc/goto-instrument/contracts/memory_predicates.cpp	84.97% <95.00%> (-0.60%)	⬇️
src/goto-instrument/contracts/contracts.cpp	94.79% <100.00%> (-0.02%)	⬇️
src/goto-instrument/contracts/memory_predicates.h	100.00% <100.00%> (ø)

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