resolvers: Use DNS resolution that supports TTL instead of arbitrary ttl.

[bwplotka]

We use https://github.com/improbable-eng/go-srvlb resolver to watch on resolver changes.

The interval of lookup is based on TTL, but std golang net libraries does not support those.

So either:

• use https://github.com/miekg/dns inside of https://github.com/improbable-eng/go-srvlb
• move to https://github.com/grpc/grpc-go/blob/master/resolver/dns/dns_resolver.go#L195