refactor startXSnap

[warner]

perform some internal refactorings to make upcoming changes simpler

• make initializeKernel async, so it can wait for xsnap bundles in record worker bundleIDs in vat metadata, use during reload #7208
• move makeStartXSnap out to a separate file, simplify it
• pass specific options/flags into makeStartXSnap, instead of the whole environment
• then make env processing the responsibility of the caller

[codecov]

Codecov Report

Merging #7235 (9b5dff0) into master (33ffd7b) will decrease coverage by 8.72%.
The diff coverage is 85.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7235      +/-   ##
==========================================
- Coverage   79.27%   70.56%   -8.72%     
==========================================
  Files         393      447      +54     
  Lines       74434    85167   +10733     
  Branches        3        3              
==========================================
+ Hits        59011    60099    +1088     
- Misses      15422    25000    +9578     
- Partials        1       68      +67     

Impacted Files	Coverage Δ
packages/SwingSet/src/controller/startXSnap.js	83.15% <83.15%> (ø)
packages/SwingSet/src/controller/controller.js	98.32% <100.00%> (+1.89%)	⬆️
...ckages/SwingSet/src/controller/initializeKernel.js	99.10% <100.00%> (+0.02%)	⬆️
...ages/SwingSet/src/controller/initializeSwingset.js	95.00% <100.00%> (+0.03%)	⬆️

... and 54 files with indirect coverage changes

[datadog-full-agoric]

Datadog Report

Branch report: refactor-start-xsnap
Commit report: a165f71

✅ agoric-sdk: 0 Failed, 0 New Flaky, 3806 Passed, 57 Skipped, 25m 43.15s Wall Time

[gibson042]

This is a reasonable refactoring. I like the movement of makeStartXSnap to its own file.