Use-after-free in DBClientConnection when handling application name metadata

The DBClientConnection::_applicationName field holds an owned string that represents the currently set value of the 'application' metadata field. In some code paths, a StringData that views that owned string is passed to a function which uses the StringData to re-write the _applicationName. That is fine, however, the code then continues to use the passed in StringData, which now refers to freed memory.