Add failpoint to allow InitialSyncTest fixture to pause initial syncing node after cloning some documents

The initial sync fuzzer currently pauses initial sync before running the 'listDatabases', 'listCollections', and 'listIndexes', for each database/collection that is being cloned. It does not, however, pause the syncing node at any time during the actual fetching of documents inside the CollectionCloner. This can prevent it from being able to deterministically reproduce certain bugs that may occur during the collection cloning process. For example, if the sync source contains a document {_id: 1}, which is cloned by the initial syncing node, and then the sync source deletes {_id: 1} and re-inserts it before the clone has finished for that collection, the syncing node may clone the document a second time. Being able to deterministically reproduce cases like this would be a helpful improvement to our initial sync test infrastructure.