network: Only insert global addresses into the DHT. (#5735)

* network: Only insert global addresses into the DHT.

Currently every address reported via libp2p-identify is inserted into
the DHT which thus contains a multitude of unreachable addresses such
as from 127.0.0.0/8 or 10.0.0.0/8.

Issue #5099 suggested a dedicated service over UDP to gauge the
reachability of an address, which would however incur extra I/O costs
and be of limited use.

As an alternative and simpler tactic, this PR only allows global IP
addresses to be inserted into the DHT unless an explicit command-line
flag `--allow-non-global-addresses-in-dht` is given or a node is
started with `--dev`. This opt-in behaviour is meant to allow
site-local networks to still make use of a DHT.

* Enable non-global in more test setups.

* Replace command-line option with different name.

* Another test fix.

substrate/Cargo.lock

@@ -2283,6 +2283,12 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "ip_network"
+version = "0.3.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2ee15951c035f79eddbef745611ec962f63f4558f1dadf98ab723cc603487c6f"
+
 [[package]]
 name = "ipnet"
 version = "2.3.0"
@@ -6417,6 +6423,7 @@ dependencies = [
  "futures-timer 3.0.2",
  "futures_codec",
  "hex",
+ "ip_network",
  "libp2p",
  "linked-hash-map",
  "linked_hash_set",