exp/cookiejar: remove external storage

This CL removes the external storage of a cookie jar
and minimized the exported API as discussed in [1].

[1] https://groups.google.com/d/topic/golang-dev/ygDB3nbir00/discussion

Update issue 1960.

R=nigeltao
CC=golang-dev
https://codereview.appspot.com/7235065

Committer: Nigel Tao <nigeltao@golang.org>
test: add test that caused a gccgo compilation failure

Updates issue 4734.

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/7228079