For production code, the ExtensionManager instance is typically

created as part of application init, because it needs to contribute to

URLs. This was causing database access to occur before apps were fully

ready, spewing warnings.
This change adds a delay_init parameter to the ExtensionManager

constructor. When this is set to True, we hold off on creating the

GenerationSynchronizer until later. For unit tests, we pass this in as

False. For real code, we pass this in as True and then the load()

method will end up finishing the initialization.

Ran unit tests.
Ran the Review Board devserver (with some matching changes to that

  codebase) and saw no more errors about early database access.