A couple of places in control.c were using connection_handle_write()
to flush important stuff (the response to a SIGNAL command, an
ERR-level status event) before Tor went down. But
connection_handle_write() isn't meaningful for bufferevents, so we'd
crash.
This patch adds a new connection_flush() that works for all connection
backends, and makes control.c use that instead.
Fix for bug 3367; bugfix on 0.2.3.1-alpha.