feat(healthcheck): HEALTH_TARGET_ADDRESS -> HEALTH_TARGET_ADDRESSES

- Specify fallback addresses
- Defaults changed from cloudflare:443 to cloudflare:443,github.com:443
- Startup check runs a parallel dial to each of the addresses specified with a global 6s timeout
- Full periodic check cycles through addresses as it fails and moves to retry

internal/configuration/settings/settings_test.go

@@ -57,7 +57,9 @@ func Test_Settings_String(t *testing.T) {
 |   └── Log level: INFO
 ├── Health settings:
 |   ├── Server listening address: 127.0.0.1:9999
-|   ├── Target address: cloudflare.com:443
+|   ├── Target addresses:
+|   |   ├── cloudflare.com:443
+|   |   └── github.com:443
 |   ├── ICMP target IP: VPN server IP
 |   └── Restart VPN on healthcheck failure: yes
 ├── Shadowsocks server settings: