pkg/client: make Describe use a POST request to the server

Otherwise it can't send Describe Rules.

Also in this commit: a bunch of new tests from when I thought I was going crazy
and trying to debug the search system, before I realized the problem was that the
client was doing a GET request and dropping most of my DescribeRequest.

Change-Id: I4ea9bed80f0d7d6b86814527b63acc3586ac1d06
camtool: add 'makestatic' subcommand

It converts a permanode set (with a bunch of camliMember) into a static-set.
Mostly for sharing a snapshot. Will likely evolve over time.

Change-Id: I9906750223a27e9a491c91f1ce2490c15d1d3986