ClientBootstrap.resolver is hard to use for single-use resolvers

Some resolvers (like NIO's internal GAIResolver) are use-exactly-once objects. They work only if used once (not 0 times, not twice).

ClientBootstrap.resolver is hard to use with those objects because one might want to pass around a pre-configured ClientBootstrap and then not use it or use it multiple times.

Maybe ClientBootstrap could offer a resolverFactory method that gets invoked exactly when we actually do the resolution.

The other option would that NIO could provide a Resolver type that wraps use-once resolvers and managed their lifetime (creates a fresh one when a resolution actually happens).