fix: Recover from a crash more gracefully

[FrederikBolding]

The current behavior for stopping Snaps (in case of crashes or idling) was prone to race conditions when requests to the Snap were dispatched around the time that the Snap crashed.

We saw this in production here: MetaMask/metamask-extension#33411

To prevent this, we can turn the stopping flag into a promise that we can await in case of multiple calls to stopSnap, but also before considering whether the Snap is started for a subsequent request.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.16%. Comparing base (79481f9) to head (ff0ab6c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3440   +/-   ##
=======================================
  Coverage   98.16%   98.16%           
=======================================
  Files         402      402           
  Lines       11123    11128    +5     
  Branches     1759     1760    +1     
=======================================
+ Hits        10919    10924    +5     
  Misses        204      204           

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