Pass UUIDs to StorageEngineImpl::_dropCollectionsNoTimestamp instead of NamespaceStrings

_dropCollectionsNoTimestamp accepts NamespaceStrings to finish drops as part of a dropDatabase command. While we believe all calls into that are correct, it would be safer to accept a vector of collection UUIDs instead of performing the NSS -> UUID lookup internally. This will help guard against any anomalies due to removing locks around DDL operations.