Don't abort collection clone due to negative document count

In CollectionCloner::_countCallback we abort the clone if the value returned from the count command on the collection is negative. However, as we document, the value returned by the count command is advisory and may not be accurate, e.g if there was an unclean shutdown. I believe we are (or should be) only using the count for advisory purposes during the clone, so we shouldn't abort the clone if the count is negative, as this can for example prevent initial sync.