Allow passing multiple fixed peers

[luisschwab]

Instead of fixed_peer being an Option<SocketAddr>, it could be an Option<Vec<SocketAddr>> so we get to have multiple fixed peers.

[Davidson-Souza]

In Bitcoin Core that's the seednode option

[luisschwab]

-seednode= Connect to a node to retrieve peer addresses, and disconnect

https://bitcoin.stackexchange.com/questions/83269/addnode-vs-seednode

I dont't want to connect to fetch addresses, but rather keep >=1 peers as fixed.

[luisschwab]

Core also allows multiple:

# Connect only to the specified node; -noconnect disables automatic
# connections (the rules for this peer are the same as for
# -addnode). This option can be specified multiple times to connect
# to multiple nodes.
#connect=<ip>