When GenerationSynchronizer put new state in cache, it used the

default expiration time for the cache backend. This is only 5 minutes by

default, unless customized in the cache settings, and then it may be up

to the actual cache server.
For a common setup, this means that the synchronizer may be invalidated

and generation bumped as often as every 5 minutes, leading to spurious

state reloads. Not always a true problem, but it can increase the

likelihood or frequency of side effects of some operations (such as

during extension reloading) when under heavy traffic.
We now set an explicit expiration time of 1 year. This can be controlled

when constructing the synchronizer, though it's unlikely the default

would need to be changed outside of load testing.

Unit tests pass.
Verified that the synchronizer was no longer expiring after 5 minutes in

local memory tests.
Also tested manually with shorter intervals and monitored events.