bus: process AddMatch/RemoveMatch driver call in proxy

Previously, AddMatch/RemoveMatch calls where processed exclusively in
the proxy. That's racy however, since subscribing to a signal might not
complete before the signal is sent due to some subsequent method call.
Hence, in order to expose the same ordering guarantees as dbus1 process
the AddMatch/RemoveMatch calls from the proxy, so that they are
dispatched synchronously to all following messages, thus fixing the
race.

Ultimately, we should probabably dissolve the driver entirely into the
proxy, as it is purely a compatibility feature anyway...