Feature: more robust updater DNS resolution

- Parallel resolver to resolve multiple hosts
- Repeat resolver to repeat resolution for a single host
- Additional parameters for fault toleration
- Do not update servers if e.g. > 10% DNS resolutions failed
- resolver package in updater package

internal/updater/resolver/resolver.go

@@ -0,0 +1,3 @@
+// Package resolver defines custom resolvers to resolve
+// hosts multiple times with adjustable settings.
+package resolver