#190 - work on factory pattern ready for v6 codebase

[timtjtim]

Motivation: basically everything about the specifics of the config schema that might change with v6 is to be handled by a specific class.

Detail: Sync does a lot less now - much more in Client and ClientV5. Slightly cleaner class file paths, but not sure if it should be x/v5/index.ts or x/xv5.ts e.g. host/v5/index.js vs host/hostv5.ts Run out of time today for e2e, integration tests, but want to get eyes on the code first before I pour more time into it in case we want to make large changes here.

To Do:

• gravity unit tests
• unit test coverage of new code
• don't like that I've split notify, but it used specifics of the host configuration 😞
• arguably the client.create method should move to the factory now
• file envvars are tied to the path of the config key, which means they've changed - I think there should be a better way to do that so this doesn't break existing installs - sort of makes keeping it v5 compatible pointless!
• one of the unit tests fails when run with yarn run test:unit but not when run with NODE_OPTIONS=--experimental-vm-modules yarn jest --runInBand test/unit/

  FAIL test/unit/sync.test.ts
    ● Test suite failed to run
  
      Jest worker encountered 4 child process exceptions, exceeding retry limit
  
        at ChildProcessWorker.initialize (node_modules/jest-worker/build/workers/ChildProcessWorker.js:181:21)
  

• as above - e2e, integration
• ?