Feat/targets to new dao

[kikito]

The new admin API is fairly similar to the previously existing one, but there are some breaking changes:

• Lists (GET /upstreams or GET /upstreams/{upstream}/targets) don't accept query-attributes for filtering any more
• Pagination: the total attribute is gone
• Format of error messages has changed to new DAO errors.
• PUT actions now are applied to entities instead of to collections. (PUT /upstreams/foo instead of PUT /upstreams
• Some error codes have changed:
  • Attempting to use an invalid name name (such an IP address) when updating an upstream has become a 500 error instead of a 400.
  • PUT actions return a 200 on creation instead of 201.
• timestamps (like created_at) are floating point numbers now (in all entities - not just upstreams and services).

New features:

• There is a attribute called next on paginated admin API results.
• When listing the targets for an upstream with db.targets:for_upstream(u), the list only includes active targets, and is reverse-sorted by created_at+id. This is the method used in /upstreams/{upstream}/targets
• There is also a db.targets:for_upstreams_raw(u) which returns all targets, including those with weight=0, associated with the upstream. That's what used in /upstreams/{upstream}/targets/all
• db.targets:for_upstream_with_health(u) is the same list as db.targets:for_upstream(u) but also includes health information (so it is slower to call). It is used in /upstreams/{upstream}/health.
• In addition to those there's db.targets:for_upstream_sorted(u) which returns the targets sorted by created_at+id. It is used internally in a couple places for things like cleaning up and building the cache.
• The db.targets:delete method creates a new target with weight = 0. The only way to really delete a target from the database is by inserting targets (this will eventually trigger a clean_history call).
• The the db.targets:post_health is the one-place for changing health status of a target. It deals with balancer changes and sending events to the cluster. It is used in POST /upstreams/{upstream}/targets/{target}/healthy|unhealthy.

Prerequisites:

• Cassandra migrations where not re-entrant (they can not "catch exceptions and continue" like postgres can). In the past the tests where run only in postgresql. I had to add a way to "ignore errors" in order to allow re-entrant migrations in Cassandra.
• There was a lurking bug related with partial updates of record-type fields.
• Needed a new kind of validator to allow validating the hash_on_header is not equal to hash_fallback_header
• The target.target attribute is an endpoint key but it is not unique. Had to remove that restriction from the system.
• Timestamps should have millisecond-precision, but until we updated the targets to the new DAO we didn't realize that we where storing them with second-precision in the database.

[hishamhm]

@kikito please remove from this PR the commits that were already merged via other PRs (I'm not doing it myself to avoid getting your local branch out of sync)

[kikito]

@hishamhm done - I have left the timestamps-as-float commit because it is needed for the tests to pass.