Implement the rangesToClean set and maintenance methods

The rangesToClean set is where the per-collection queue of chunk ranges to be cleaned is stored. This task is for introducing this set under the CollectionMetadataManager class, adding the appropriate methods for its maintenance. In particular:

• addRange(ChunkRange range) - adds a new range to be cleaned up. The newly introduced range must not overlap with the existing ranges.
• removeRange(ChunkRange range) - removes the specified range from the ranges to be cleaned up.