We don't always want to immediately write new connections to disk, to
facilitate "runtime" or "temporary" connections where an interface's
runtime config isn't backed by on-disk config.  Also, just because
an interface's configuration is changed doesn't necessarily mean
that new configuration should be written to disk either.

Add D-Bus methods for adding new connections and for updating existing
connections that don't immediately save the connection to disk.

Also add infrastructure to indicate to plugins that the new connection
shouldn't be immediately saved if the connection was added with the
new method.