The Server struct has exported fields, which allows users to manually
create a Server object without using using NewServer or NewTLSServer
and directly call Start or StartTLS on their object.

In order to ensure that manual creation of Server works, the
NewUnstartedServer function should not initialize Server in any way
that the user was not able to do themselves. For example, the setting
of a unexported filed, client, is not something a user can do.
Thus, rather than setting the client field in NewUnstartedServer,
we lazily initialize it when Start or StartTLS is called.

Otherwise, the Server logic can nil panic later when it assumes that this
field has been initialized.

Fixes #20871

Change-Id: I65c6a9f893ea963b0fbad0990b33af08007c1140
Reviewed-on: https://go-review.googlesource.com/47353Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>