Add isc_nm_read_stop() and remove .reading member from ccmsg

Add isc_nm_read_stop() and remove .reading member from ccmsg

We need to stop reading when calling isc_ccmsg_disconnect() as the
reading handle doesn't have to be last because sending might be in
progress.  After that, we can safely remove .reading member because the
reading would not be called after the disconnect has been called.

The ccmsg_senddone() should also not call the recv callback if the
sending failed, that's the job of the caller's send callback - in fact
it already does that, so the code in ccmsg_senddone() was superfluous.