Freeze server/server.go for deprecation

* Add notice in server/server.go explaining not to add stuff there, and
what to do instead.

* Make myself a sole maintainer of server/ to avoid concurrent edits
while refactoring.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Merge pull request #5334 from shykes/freeze-server

Freeze server/server.go for deprecation