Uniqueness and idempotency

[tupui]

Event records are unique and we should prevent double ingestion.

Evaluate:

• Composite primary keys
• Surrogate index
• Uniqueness on multi-columns
• Other?

[tupui]

Closed by f6e2c52f3357533032fa1564401620e92cd93916

I went with uniqueness on multi-columns to keep primary key logic simple for every DB consumer. This has an impact on write performance but we should be good for a while before having to do fancier things (e.g. partition by ledger could solve it all quite easily.)