New PIA servers support (#227)

* Adapt storage: SyncServers write to file option, export FlushToFile
* CLI built-in updater for old and new PIA servers
* Update hardcoded IP addresses for PIA old and new servers
* Add PIA old to allServers struct and update timestamps
* Adapt code to work with new and old PIA servers
* Remove PIA subdomains (unneeded) from resolver tool

internal/updater/options.go

@@ -0,0 +1,8 @@
+package updater
+
+type Options struct {
+	PIA    bool
+	PIAold bool
+	File   bool // update JSON file (user side)
+	Stdout bool // update constants file (maintainer side)
+}