fixPreviousTxnID: add sfPreviousTxnID/sfPreviousTxnLgrSequence to all ledger objects

[mvadari]

High Level Overview of Change

This PR adds sfPreviousTxnID/sfPreviousTxnLgrSequence as fields to all ledger objects that don't already have them (except LedgerHashes, because that seems a bit redundant and also those objects aren't modified via transaction).

The affected objects are:

• DirectoryNode
• Amendments
• FeeSettings
• NegativeUNL
• AMM

Context of Change

It makes it a lot easier to go through the history of a ledger object. Related to #4602.

Type of Change

• 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 not work as expected)

Test Plan

Tests have been added to ensure that everything behaves as-is prior to the amendment and transitions seamlessly when the amendment is activated.

[mvadari]

@Silkjaer @RichardAH @nixer89 @dangell7 this is ready for review now.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.19608% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 76.97%. Comparing base (c88166e) to head (c8af202).

Files	Patch %	Lines
src/test/ledger/Directory_test.cpp	92.68%	0 Missing and 3 partials ⚠️
src/ripple/ledger/impl/ApplyStateTable.cpp	33.33%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4751   +/-   ##
========================================
  Coverage    76.97%   76.97%           
========================================
  Files         1129     1129           
  Lines       131914   131959   +45     
  Branches     39629    39687   +58     
========================================
+ Hits        101539   101574   +35     
- Misses       24459    24505   +46     
+ Partials      5916     5880   -36     

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[mvadari]

Proposed commit message:

fix amendment to add `PreviousTxnID`/`PreviousTxnLgrSequence`

This amendment, `fixPreviousTxnID`, adds `PreviousTxnID` and
`PreviousTxnLgrSequence` as fields to all ledger objects that did
not already have them included (`DirectoryNode`, `Amendments`,
`FeeSettings`, `NegativeUNL`, and `AMM`). This makes it much easier
to go through the history of these ledger objects.

[ximinez]

Because I can't help myself, I made all the changes from my feedback, except the feature name, in a branch: mvadari/rippled@fix-prev-txn-id...ximinez:rippled:mv/4751-prev-txn-id

Pick and choose as you see fit.

[Bronek]

Looking good !

[ximinez]

🎉

[mvadari]

I'm not sure who to tag to indicate that this is ready to go (maybe @seelabs) - the Mac test failure looks like the known flakiness, not an actual failure.

[ximinez]

I see you proposed a commit message (#4751 (comment)), and I just added the "Passed" label. One of us will merge it soon.

[intelliot]

API Change: this adds fields to objects. I don't think it's a breaking change. But it would still make sense to update the models in the client libraries (@mvadari has noted this).