Persistent sockets aren't very persistent

Persistent sockets will work when reused in the same page request. However after a page request completes, it appears the persisted socket is closed, and thus a new page request on the same thread will be unable to use the persisted socket and a new connection will be created.

It appears this is sweep's fault. I think File and Socket's implementation of sweep is doing too much. They'll call closeImpl on the data object which will close the file/socket, but that will also happen in the data object's destructor, so I think we can remove that extra call to closeImpl and just rely on that happening when the data object's refcount hits zero.