We have some classes (SiteConfiguration and ExtensionManager) that

need to synchronize an ID representing a generation of their state

across processes, so that if one process changes some state, the others

will reload from the database. We've left it up to those classes in the

past to do this, but that meant logic and code was getting duplicated,

and any future classes that needed this behavior would have to duplicate

it as well.
This introduces a GenerationSynchronizer class, which handles the

synchronization and expiration checking of a generation ID. This can

easily be used by classes that need this sort of behavior, without

having to add a lot of new code.
SiteConfiguration and ExtensionManager have been updated to use this.

All unit tests pass.
Played around with the functionality in Review Board. Didn't see any

obvious breakages.