Before the alwaysCompressedBuckets project, we would only compress buckets upon closing them. To keep track of these buckets we would store them in the ClosedBuckets vector. However, now that the project is released, we do not actually use this struct anywhere. 

We can remove it from the following:

bucket_catalog_internal::doRollover
bucket_catalog_internal::closeOpenBucket
bucket_catalog_internal::closeOpenBucket (the other implementation)
bucket_catalog_internal::archiveBucket
bucket_catalog_internal::expireIdleBuckets
bucket_catalog_internal::reopenBucket

We should also be able to remove all higher-level references to it, remove it as a member of InsertContext and SuccessfulInsertion, and then finally get rid of the class entirely.