SERVER-39165 introduced the waitForFailPoint command and corresponding fail_point_util.js library. We should update initial_sync_update_missing_doc.js to use the fail point helpers because a slow checkLog command can cause a deadlock in tests that use the initial_sync_update_missing_doc library.

This is because we will acquire the global lock in IS mode when trying to log a slow command. If we have turned on a failpoint (for example, initialSyncHangBeforeGettingMissingDocument) that has already acquired a lock in X mode, this attempt to log the slow command will hang.