Integrate CancelableOperationContext into ReshardingOplogFetcher

Note that the OperationContext constructed within the callback given to Shard::runAggregation() cannot use the shared_ptr<executor::TaskExecutor> given to ReshardingOplogFetcher::schedule() for the CancelableOperationContext. This is because all of its threads may already be consumed waiting for the Shard::runAggregation() task being run on the fixed executor to finish. See also the CancelableOperationContextFactory class mentioned in SERVER-55306.