Bug 922147 - Pre: Remove some GlobalSession input checking.
Bug 922147 - Part 1a: Add getAuthHeaderProvider.
Bug 922147 - Part 1b: Remove CredentialsSource.
Bug 922147 - Part 1c: Factor out SyncConfiguration.getClusterURL.
Bug 922147 - Part 2a: Make SyncConfiguration.{username,password} final.
Bug 922147 - Part 2b: Remove SyncConfiguration.password.
Bug 922147 - Part 3: Replace passwords with AuthHeaderProvider.
Bug 922147 - Post: Remove SyncConfiguration.userAPI.