Add safeguard when reading timestamp from cf_promises_valid.

If the file contains valid JSON, but is missing the timestamp field,
then the attempted fetch of the value will crash, leaving all daemons
dead in the water.
Separate validated and release_id information into separate files.

The base problem is that only the policy server has enough information
to generate the release_id, most notably because the server will have
a git repository with a tag, whereas each agent will not. Therefore
the agents will generate a different release_id, based on file
contents. Add to the problem that stale files at each agent can also
influence the result, and you have a mess. Therefore only the policy
server should be allowed to generate the release_id.

This is a problem because each agent must be able to generate
cf_promises_validated, so that it can trigger a policy reload in its
daemons.

The problem is solved by separating those two pieces of information
(timestamp in cf_promises_validated, and releaseId in
cf_promises_release_id). All agents generate the former; only the
policy server generates the latter, with other agents downloading it.
Merge pull request #1325 from kacfengine/separate_release_id_file

Separate validated and release_id information into separate files.