client/authority-discovery: Compare PeerIds and not Multihashes

In order to tell whether an address is the local nodes address the
authority discovery module previously compared the Multihash within the
`p2p` Multiaddr protocol.

rust-libp2p recently switched to a new PeerId representation (see [1]).
Multihashes of the same PeerId in the new and the old format don't
equal.

Instead of comparing the Multihashes, this patch ensures the module
compares the PeerIds

[1] https://github.com/libp2p/rust-libp2p/issues/555