Source of truth for transaction errors (index vs. event stream)

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We assume that replica add and remove actions are specified as deltas against a published document since currently the only mechanism available in the STAC Transaction API to update alternate assets is an RFC7386 PATCH. To apply a PATCH to a document in a distributed system we must choose some source(s) of truth (or state) against which that delta will be applied and think about how to resolve discrepancies if the update must be applied to multiple sources of truth in different parts of the system.

There are multiple ways to design a system that can resolve partial updates, we'll outline two here that we think we have reasonable paths to implement and discuss some of the trade offs made with each design.

GOAL: Decide which approach we'll take in the ESGF-NG architecture.

[lliming]

PR #25 provides two options, including sequence diagrams and pros/cons for each.