MAGE-1110: Indexing Queue updates

[damcou]

This PR contains:

• Some cleaning on the Queue and Job models
• A new way of calculating maximum batch size in the queue, now we take into account the values defined at store level instead of the default one. This allows to take the values set by the batching optimizer into consideration.
• Added some missing store scopings
• Fixed a merge test

[cammonro]

Nice progress on cleaning up this code! I do have a few concerns if you wouldn't mind taking a look. Additional comments in Jira.

[cammonro]

Further discussion in Jira detailing possible follow up action items but I'm ok with this iteration of the code. Approved!