Change the cache expiration policy to reduce GC

The 'expireAfterWrite' expiration policy is causing excessive GC during
high load. Since we're going to be seeing the same things over and over
again, it is better to expire things from the cache when we stop seeing
it. This will reduce the amount of GC.
Update insertedLocators comment to reflect new expiration policy
Merge pull request #387 from dlobue/dominic/change_cache_expiration_policy_to_reduce_gc

Change the cache expiration policy to reduce GC