Djblets 5 introduced handling for cache backend errors, attempting to

avoid outages if the cache server was down or misconfigured. However, it

had a flaw in that if GenerationSynchronizer experienced a backend

failure when checking for state expiration, it would determine that the

state has not expired.
This prevented cache server changes from taking effect. If cache

configuration was misconfigured, all servers/threads/processes would

inherit the broken state and then assume it never expires, preventing

any fixes from being loaded.
Now, we assume the state is invalid in this case, and force refreshing

state, hoping to load in corrected state.
We also now always set a sync_gen state when attempting to

fetch-or-create a generation value, rather than keeping a stale value

(which may be tied to bad state).
This doesn't fully solve issues with broken cache state, as Django

doesn't sandbox cache backend failures at all, and will break during

session loading (and other places). However, this does offer a

possibility of fixing that issue, provided cache settings state is reset

before a user session or other impacted Django code runs, if some

process can update siteconfig settings for cache configuration through

another means.

Unit tests pass.