state: isolate/generalise refcounting

[fwereade]

Refcount values are (1) implied by the rest of a model description and
(2) pure implementation details necessitated by mgo/txn. They've been
dropped from description.Application; refcount sanity is now maintained
implicitly during import by the add*Ops-level funcs.

Refcount operations themselves have been extracted to refcounts_ns.go
...and the "settingsrefs" collection has been renamed to "refcounts",
reflecting its broader purpose.

Charm refcounts have been added, but nothing currently holds references
to them.

This is identical to PR5939, except that nsRefcounts.JustRemoveOp now accepts a negative value to indicate "don't assert anything, just kill it". To QA, deploy ~/containers/bundle/elk-stack and wait for all machines to be started; then destroy-controller --destroy-all-models, and check that everything gets cleaned up.

[voidspace]

The deploy isn't clean, which probably doesn't matter for this specific branch. There's a problem with kibana:

kibana/0 error idle 2 54.196.108.173 80/tcp,9200/tcp hook failed: "rest-relation-changed" for elasticsearch:client
logstash/0 blocked idle 3 54.197.94.74 Missing JRE
zulu8/0 error idle 54.197.94.74 hook failed: "java-relation-joined" for zulu8:java
unit-kibana-0: 12:47:32 INFO worker.uniter.jujuc running hook tool "relation-get" ["private-address" "-"]
unit-kibana-0: 12:47:32 INFO unit.kibana/0.rest-relation-changed error: "-" is not a valid unit

[voidspace]

Deployed to AWS. Both kill-controller and destroy-controller (two separate deploys) worked fine.

[fwereade]

Thanks Michael. Sorry the instructions weren't clear: the point was indeed to safely tidy up non-clean deploys.

[voidspace]

LGTM

[fwereade]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/8873

[fwereade]

mongo failures; $$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/8876

[fwereade]

mongo again; /sigh; $$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju