The /api/flamenco/{manager-id}/task-update-batch endpoint is extended to
return a list of task IDs that should be cancelled by the Manager. The
endpoint now also accepts an empty list of task updates.

This makes it possible for other modules to use gocheck too, without
running the tests multiple times.

This endpoint is used by Workers to determine whether they are allowed to
keep running their current tasks. When the task is no longer in a runnable
state (""active", "claimed-by-manager" and "queued" are runnable) or
assigned to a different worker, the answer is "no".

Also simplified some code; after sending a task to a worker, the Manager
no longer sends a PATCH command to Server to change the task status. The
worker is responsible for setting the task status to "active", since those
status changes are nicely batched up and retried if necessary.

This includes checking for any server-side changes. If there are, the
queued task is updated accordingly, and the queue is re-examined.

Connection errors, redirects and Server-side internal server errors are
ignored, and the task is considered "unchanged" in those cases. Errors
in the 400-range (Not Found, Forbidden etc.) result in the task being
marked as "canceled" in the queue. The task is NOT removed from the
queue, so that the information about the task being unavailable to workers
is explicitly available.

Furthermore, this commit introduces gocheck for unit testing, which gives
us setup and teardown functions.

This also provides us with functions to call when the Manager tells us to
stop the currently running task.

Only logged at debug level. Copied code from the Blender Cloud Add-on,
where it has also been proven useful.

The worker now first aborts any task-fetching task, then queues up any
remaining task updates, then flushes that queue to the Manager, before
finally shutting down.

The job status can change via JobManager.set_job_status() or by PUTting
a job document with a changed status.

The compiler is now actually registering, and the 'format' parameter
is optional. This also required the frame_range_merge() function to
return unicode strings (which is a good idea anyway).

Its meaning was similar to 'claimed-by-manager', which was found to be
more descriptive.

For now, only the job status changes; this doesn't yet cause ripple effects
to the statuses of other tasks belonging to the job.

This function sets the status field of a document in MongoDB to some value,
without fetching the entire document, and without going through Eve. This
is faster, and avoids Eve's permission system (permission is already
known through our own logic).

This thus overwrites the activity string of the last-executed command.

Before this comment, logs & activities would be queued, and this queue was
pushed to master when a log or activity was registered and certain criteria
were met. As a result, when no more logs/activities were registered, the
queue would not be flushed. This is now solved by scheduling a push to
master, and cancelling that task if that push happens for any other reason.

For now we parse the output of Blender and return it as a formatted
string for the 'activity' field. At some point we may want
to consider this as a subdocument of the task (instead of a string field).

Of course this only happens if there were any activity updates at all.