acme: add Client.WaitAuthorization for polling

It is quite often the case users need to poll an authorization until it
reaches a final state.

This change adds a handy method to do exactly that,
as well as updating autocert.Manager to use the new method.

Change-Id: I4d25e4ee751731815b77980caab6da98d3440b5d
Reviewed-on: https://go-review.googlesource.com/27431
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>