gddo-server/poller: add package poller

Copied from
https://go.googlesource.com/pkgsite/+/cb9361e611fe25baabab637bc260f48b345bd1da/internal/poller

Change-Id: I02fe11b8ef1be38af705038c8ff5ef1c945c4f6c
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/285832
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>