* Multi Dependency Support - Registration & Enqueue Call
Internal API changes to support multiple dependencies.
* Store all of a job's _dependencies_ in a redis set. Delete that set when a job is deleted.
* Add Job#fetch_dependencies method - which return all jobs a job is dependent upon and optionally _WATCHES_ all dependency ids.
* Use Job#fetch_dependencies in Queue#call_enqueue. `fetch_dependencies` now sets WATCH and raises InvalidJobDependency, rather than call_enqueue.
`Queue` and `Job` public APIs still expect single ids of jobs for `depends_on` but internally register them in a way that could support multiple jobs being passed as dependencies.
Next up: need to update Queue#enqueue_dependents
* Use existing fetch_many method to get dependencies.
Modify fetch_dependencies to use fetch_many.
* Remove default value for fetch_many's connection parameter
* PR review housekeeping
* Remove a duplicate test
* Oneline something
* Fix missing colon in dependencies key
* Delete job key, dependents and dependencies at once
* More Fixes From Code Review
Updates to Job, Queue and associated tests.
* When Checking dependencies Avoid, trip to Redis
* When checking the status of a job, we have a 'clean' status of all dependencies(returned from Job#fetch_dependencies) and the job keys are WATCHed, so there's no reason to go back to Redis to get the status _again_.
* Looks as though, the `_status` set in `Job#restore` was bytes while it was converted to text(`as_text`) in `Job#get_status` - for consistency(and tests) converting to text in `restore` as well.
* In `Queue#enqueue_call`, moved WATCH of dependencies_key to before fetching dependencies. This doesn't really matter but seems more _correct_ - one can imagine some rogue API adding a dependency after they've been fetched but before they've been WATCHEed.
* Update Job#get_status to get _local_ status
* If refresh=False is passed, don't get status from Redis; return the value of _status. This is to avoid a trip to Redis if the caller can guarantee that the value of `_status` is _clean_.
* More Fixups
* Expire dependency keys in Job#cleanup
* Consistency in Job#fetch_dependencies
* Convert `_dependency_id` to `_dependency_ids`
Change `Job`s tracking from a single id of it's dependencies from a single _id_ to a list of _id_s. This change should be private to `Job` - especially leaving `Job#to_dict` and `Job#restore`s treatment of a single 'dependency_id' intact.
This change modifies existing tests.
* Remove reliance upon dependency property in tests
... use dependency.id not `_dependency_id`
* Re-add assertions for Falsey Values
* Add _dependency_id property
For backwards compatibility with other libs such as django-rq and rq-scheduler
* add doc page for job registries
* Use consistent language in registry type descriptions
* Correct usage of 'connection' parameter in Job Registry doc