Add mutex to getSnapState to prevent concurrent decryption

[Mrtenz]

This wraps getSnapState in a mutex, to ensure we only decrypt once when calling it at the same time. The first call will decrypt the state and cache it in the Snaps runtime, and the next call will just use the cached result.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.93%. Comparing base (adecea5) to head (40f1110).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3234   +/-   ##
=======================================
  Coverage   94.93%   94.93%           
=======================================
  Files         508      508           
  Lines       11219    11220    +1     
  Branches     1730     1730           
=======================================
+ Hits        10651    10652    +1     
  Misses        568      568           

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