Support for Safe Secondary Reads requires assistance from CollectionShardingState subsystem, including MetadataManager and CollectionRangeDeleter.

Specifically:

• ScopedCollectionMetadata::operator==, !=
• CollectionShardingState::overlappingMetadata(ChunkRange)
  -> std::vector<ScopedCollectionMetadata>
• CollectionRangeDeleter to write a document StartRangeDeletion to collection admin.system.version containing

    { _id: 'startRangeDeletion', ns:<collection>, epoch:<epoch>, min:<low>, max:<high> }