Drop pending reaper must not delete the _dropPendingNamespaces entry until after the drop occurs

Otherwise waiters on the drop will proceed before the drop finishes. Waiters call getEarliestDropOpTime() to determine until what optime they should wait, and getEarliestDropOpTime() looks at _dropPendingNamespaces to determine that.

getEarliestDropOpTime() can currently see that _dropPendingNamespaces is empty, when there are still pending-drops to be completed, and return boost::none – or some misinformed answer --, which tells the caller that there's nothing for which to wait.

Discovered while testing SERVER-33244's patch on no_implicit_collection_creation_in_txn.js