The Manager will return a 409 Conflict status for updates to tasks that
have been assigned to another worker. This change prevents the worker
from trying to push these updates indefinitely to the Manager.

The task timeout checker runs periodically, but only starts 5 minutes after
manager startup. This allows workers to push updates to the manager after
downtime, before getting timed out. This /timeout URL is there to allow
enforcing timeouts within that 5 minute window.

MergeExrCommand is now called MergeProgressiveRendersCommand, to be
consistent with the job compiler on Server.

Now tasks can be sorted by job and task priority, and different tasks can
have different priorities within a job. This will allow for better control
over task ordering than just relying on parent relationships.

Tasks are sorted by job priority first, and then by task priority.

This is quite useful if one task failed (for example when a worker times
out) and you want to re-queue it without cancelling any currently active
tasks.

When 'src' needs to be moved to 'dst', but 'dst' already exists, the
command finds a suffix that doesn't exist yet.

This improves performance when dealing with large graphs.

As a result, fetchTaskFromQueueOrManager doesn't need the http.Request()
object pointer any more.

Works on files, directories, and non-existing paths.

The render output directory is moved to the same name, with its
modification timestamp appended in ISO-8601 format.

As a result, the render output directory MUST be known to Flamenco, and
thus from now on is a mandatory setting for the blender-render job type.

For now this is intended as a debug tool. It uses vis.js to visualise the
dependency graph of the job.

We need a format that can handle sub-second precision, which is not
provided by the HTTP date format (RFC 1123). This means that we can't use
the Last-Modified header, as it may be incorrectly interpreted and
rewritten by HaProxy, Apache or other software in the path between client
& server.