Refresh theme-liquid-docs on theme-check startup

[mgmanzella]

Summary

Closes #651

Refreshes docs async on theme-check startup, after discussing with @karreiro we decided it's out of scope to periodically refresh while theme-check is running

Since this is a medium size PR, I broke this down into 4 commits:

• Add async download in SourceManager#refresh and mechanism to notify SourceIndex liquid schema is out of date
  • SourceManager#download_or_refresh is called in on_initialized to download docs async while theme-check loads the existing outdated docs
  • SourceManager#refresh downloads updated docs to a temp directory in case the download fails
  • SourceIndex::FilterState, SourceIndex::ObjectState, and SourceIndex::TagState are used to notify SourceIndex if a schema is out of date after refresh
  • SourceManager marks each liquid schema variable's respective state to "out of date" when updated docs are successfully downloaded
  • SourceIndex marks each liquid schema variable's respective state "up to date" when it finishes loading it
• Tests for SourceIndex state classes and SourceManager#refresh, create helper for shared stubs
  • New tests for new state classes and a small refactor to abstract source stubbing into its own helper
• Cleanup -- fix naming from test/shopify_liquid/scope_index to test/shopify_liquid/source_index/
  • Some cleanup since I was already modifying files in this directory, just changes the test directory for source_index class tests
• Remove class method mock (Net::HTTP.any_instance), affects other tests
  • Change to fix flaky tests that were failing because of mocking any_instance of Net::HTTP

Tophatting Steps

• In your terminal run rake download_theme_liquid_docs if you dont have the version of theme-liquid-docs with a revision file
• In your local copy of theme-check, modify SourceManager#refresh to add sleep(30) in the thread
• Modify the revision file at data/shopify_liquid/documentation/latest.json to change the revision key to "out_of_date_sha"
• Modify the local copy of your docs to change the documentation for a particular object (eg modify collections documentation)
• Restart theme-check and open a theme
• Confirm autocomplete still works
• Confirm the docs are downloaded, the revision is updated in latest.json, and the docs are updated in theme-check when using autocomplete