Don't serialize PeerPtr value in RPC connections

PeerPtr is used in RPC signatures for enabling receivers to send replies
to a particular peer rather than broadcast to all connected ones.
To enable this, the SignalProxy transparently replaces the bogus value
received over the network with the actual address of the local Peer
instance.

Because the actual value isn't needed on the wire, it should be
serialized as null. This also prevents the accidental use of a bogus
remote pointer address.