Switch ReshardingCollectionCloner to run on a separate TaskExecutor

The changes from ff6685c as part of SERVER-49787 had the ReshardingCollectionCloner run on the current thread for convenience with the testReshardCloneCollection command. Switching the ReshardingCollectionCloner to run on a separate TaskExecutor is a prerequisite to integrating the ReshardingCollectionCloner into the RecipientStateMachine. The RecipientStateMachine's thread needs to react to the mongo::Futures returned from multiple components (including ReshardingCollectionCloner) that are running concurrently.