- Convert closures into methods

 - Remove the `profileFetchRetryOpts` field that was used only to shorten the
   delay between retries in tests, in favor of mocking
   `retryPromiseOperation` in tests to remove the delay between retries.

 - Change `SessionService` instance creation in tests to allow
   individual tests to run custom setup logic before the service is
   constructed. This was needed due to allow the `serviceConfig` mock to
   take effect when the `SessionService` constructor runs

 - Remove unnecessary custom Sinon sandbox in tests