add rpc_public_addr_subnet config option

In case `rpc_public_addr` is not set, but autodiscovery is used, this
allows filtering the list of automatically discovered IPs to a specific
subnet.

For example, if nodes should pick *their* IP inside a specific subnet,
but you don't want to explicitly write the IP down (as it's dynamic, or
you want to share configs across nodes), you can use this option.

Cargo.toml

@@ -55,6 +55,7 @@ hexdump = "0.1"
 hmac = "0.12"
 idna = "0.5"
 itertools = "0.12"
+ipnet = "2.9.0"
 lazy_static = "1.4"
 md-5 = "0.10"
 mktemp = "0.5"