Take a shared_ptr to Trackers in IndexBuildsCoordinator before calling waitUntilNoIndexBuildsRemain

We have memory errors in the IndexBuildsCoordinator's use of the CollectionIndexBuildsTracker and DatabaseIndexBuildsTracker objects where we call waitUntilNoIndexBuildsRemain through hash map iterators rather than acquiring a safe shared_ptr to call wait upon, leading to a use after free when the entry is removed from the hash map and the caller of waitUntilNoIndexBuildsRemain does not have a shared_ptr instance to keep the object in memory.