use the Srv.end callback for freeing the srv and closing the
file descriptor of a connection. this makes sure we wont free
the srv while there are still outstanding requests that would
access the srv when doing the respond() call.
in multithreaded programs, we have to wait until all outstanding
requests have been responded before closing down the srv.
dont make write errors sysfatal(), only print them. in case if
listensrv() is used we dont want to exit the process in respond()
called by some worker thread.
make sure Tversion is only handled when there are no outstanding
requests and make sure message size is sane.
