Optimise next prev record

[RealOrangeOne]

Description

Improve performance of next_record and prev_record.

Motivation and Context

Using .instance will run an additional query to get the value for fields which are excluded from history. These properties don't need that, they only need _meta, which is accessible from the .instance_type without running any queries.

By executing fewer queries, the properties get faster. This can be especially important if the columns contain a large amount of data.

How Has This Been Tested?

I've manually applied this patch to a project, but also added relevant unit tests (and fixed an existing one which wasn't running).

Screenshots (if appropriate):

N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have run the make format command to format my code
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have added my name and/or github handle to AUTHORS.rst
• I have added my change to CHANGES.rst
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #791 (31b9374) into master (c5943f4) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #791      +/-   ##
==========================================
+ Coverage   97.68%   97.69%   +0.01%     
==========================================
  Files          19       19              
  Lines         994      999       +5     
  Branches      151      151              
==========================================
+ Hits          971      976       +5     
  Misses         10       10              
  Partials       13       13              

Impacted Files	Coverage Δ
simple_history/manager.py	97.59% <100.00%> (-0.06%)	⬇️
simple_history/models.py	97.86% <100.00%> (ø)
simple_history/utils.py	100.00% <100.00%> (ø)

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[RealOrangeOne]

Unfortunately it seems this causes issues where history of different models ends up in prev_record etc

Fixed in e250831

[jeking3]

I applied the tests to 3.0.0 (master) and found them to be failing with too many queries, when I applied the changes in the pull request they correctly reduced.