frequency cannot be configurable for DNS resolver

[kazegusuri]

DNS resolver has fixed duration for DNS resolution (30 min) for now.
I want to customize it as smaller duration to resolve more frequently.

dnsBuilder it self has freq field for resolution, but there is no interface to change it.
resolver.ResolveNowOption is the common option for resolvers.
We can populate duration by the option, but is it good for the option?

What version of gRPC are you using?

v1.15

What version of Go are you using (go version)?

go version go1.11 darwin/amd64

What operating system (Linux, Windows, …) and version?

Linux, Mac

What did you do?

What did you expect to see?

I want to customize frequency of DNS resolution

What did you see instead?

fixed duration is used for resolution

[dfawley]

To be in-line with the other grpc implementation languages, we will be removing polling from the DNS resolver; updates will be only performed when a connection error occurs.

See #1663 for more information.