There needs to be a way to limit the size of the InMemoryDatastore. At minimum we need to be able to support: remove on fetch (with option of restricting to specific datasetTypes), and keep most recent N. It would also be useful to be able to expire based on size but afw Exposure objects do not yet have a way of reporting how large they are.

One complication is understanding how removal from the datastore affects butler registry. For a ChainedDatastore this is relatively straightforward if the dataset was persisted by one of the other datastores.