Send HighestVersion attribute to Inflator

[HebaruSan]

Motivation

Now that #101 has given us the ability to compare module versions in Python the same as in CKAN-Core, this is the Scheduler + Webhook part of KSP-CKAN/CKAN#2824, which aims to reduce the need for manual maintenance of x_netkan_epoch properties.

Changes

A new CkanGroup class represents all of a given module's .ckan files in a CKAN-meta repo. This class provides a highest_version function that parses the .ckan files and compares their version properties to find the highest one, and returns it.

The Ckan.sqs_message function now accepts a CkanGroup parameter and passes it to a new sqs_message_attribs function to generate the MessageAttributes property of the message. Naturally, this uses CkanGroup.highest_version to set HighestVersion.

If x_netkan_allow_out_of_order is set to true in a netkan, this means auto-epoching is turned off, so the HighestVersion property will not be set and its .ckan files will not be parsed.

The Scheduler now checks out CKAN-meta and uses it to pass CkanGroup objects to Ckan.sqs_message. The Webhooks also pass CkanGroup objects to Ckan.sqs_message.

Fixes #7.

[techman83]

This technically isn't related to the scheduling component, but what's this going to look like on the indexing side of things? I'll refresh myself with the other PR.

[HebaruSan]

The Indexer will not need any changes for this; it's solely a Scheduler+Inflator change.

[techman83]

Yep, after refreshing myself I realised we could re-utilise the staging logic without any changes to the indexer. Which is rather neat.

[techman83]

Awesome. Love your work!