During recovery, we want to initialize the in-memory size/count for each collection in the catalog by combining the persisted data in the config.fast_count_metadata_store with the persisted oplog
entries after the valid-as-of timestamp. This work should enable constant time access of collection size/count by storing the size/count directly within either the CollectionImpl or the WiredTigerRecordStore as two atomics.

To initialize the in-memory metadata, we use the following algorithm. After the last WiredTiger checkpoint has been loaded and the oplog has been replayed,
1. Initialize a map of Collection UUID -> CollectionSizeCount
2. Iterate over the config.fast_count_metadata_store and insert each collection's size and count into the map
3. Read the valid-as-of timestamp
4. Use aggregateSizeCountDeltas() to accumulate the changes to size/count since the valid-as-of timestamp
5. Combine the aggregateSizeCountDeltas() map with the map created in step 1.
6. Write the combined size/count to its respective collection in the collection catalog. Note that the collection catalog uses copy-on-write semantics with an immutable map storing each collection, so doing this swap in one operation would be nice.

Also consider the space complexity of steps 1 + 4. Right now, aggregateSizeCountDeltas() is inteded to be used before readAndIncrementSizeCounts(), but we want the opposite ordering here: for all entries in the size count store, if there exists an entry in the oplog, increment it.