Allow local peer connections on Regtest

[arya2]

Motivation

This will be needed for the RPC tests and to match zcashd's behaviour on Regtest as closely as possible.

Design

On Regtest:

• Return the configured initial testnet peers without loading the peer cache in zebra_network::Config.initial_peers() instead of returning an empty set, and
• Reject incoming connections and avoid attempting outbound connections with peers outside the local host domain.