critical fix: #378 calculate ondex from prior est evt, not proposed one

[kentbull]

Problem

Failed multisig rotation where a new member is added and a new one is dropped during the same rotation. The root cause is the incorrect calculation of the ondex, the index of the prior next key digest, from the proposed rotation event, from rstates passed in to GroupIdentifierManager in the constructor. rstates is for the proposed event, not the prior establishment event.

Solution

Pull the n next keys (prerotated committed keys) from the prior establishment event and calculate the ondex values from that rather than the rstates from the proposed event.

Background

KERIpy validates ondexes, which are the pre-committed next key digest indexes, from the prior establishment event, not the new, proposed establishment event.

The reason we haven't hit this in the past is because all of the existing tests never swapped in a new member in the index/ondex of a prior member. We always had a clean addition of a new member and then a separate operation for the removal of the to-be-deleted member.

Essentially, we didn’t hit it because the existing tests never exercise departing-current-signer signs while absent from proposed rstates (the next set of pre-committments).

See #378 for an example rotation that reproduces this bug.

[kentbull]

This also adds a fair bit of documentation to code that was poorly documented.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.53%. Comparing base (c87e99b) to head (0de697f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #379   +/-   ##
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  Coverage   97.53%   97.53%           
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  Files           1        1           
  Lines         325      325           
  Branches       24       24           
=======================================
  Hits          317      317           
  Misses          8        8           

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