Cache snap state in memory

[Mrtenz]

To improve performance when using state, especially when using the new state methods (#2916), we now cache state in memory, and write updates to disk asynchronously. The first time the state is fetched, it's cached in the Snap's runtime data, which is used in subsequent calls.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.73%. Comparing base (5159929) to head (3916cc7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2980   +/-   ##
=======================================
  Coverage   94.72%   94.73%           
=======================================
  Files         497      498    +1     
  Lines       10826    10854   +28     
  Branches     1654     1659    +5     
=======================================
+ Hits        10255    10282   +27     
- Misses        571      572    +1     

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