Document consumption modes

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When consuming via Kafka, the high level modes include:

Representation	Description	Implemented In	Notes
Serial Messages	Messages are individual events tagged with a key. No parallelism is or can be employed.		Some current usage uses this pattern but should be deprecated
Batched	Consume, processing batches for a given partition together. Checkpoint when batch completed	Jet.ConfluentKafka.FSharp
Stream Event Spans	Messages are spans of events from a specified Stream Name (also the Key), with a specified index	Propulsion.Kafka	Implemented in StreamedConsumer and proProjector + proConsumer template. Allows production in parallel without max.in.flight=1. Consumer deduplicates
Stream Summary Events	Messages contain a rendition of a summary as at a particular version. Version is monotonic and consumer is free to / should only use the most recent one		TODO build a dotnet-templates example and provide default wiring in StreamedConsumer. Consumer can deduplicate by taking highest version (probably tracking last-seen per stream to deduplicate). Ref http://verraes.net/2019/05/patterns-for-decoupling-distsys-summary-event/
Ordered Summaries	Messages represent state at a point in time. The topic's ordering within a key defines the 'version'. Consumer needs to group by key, dropping all but the newest. There's no way to de-duplicate double sends.		Stream Summary Events are preferred, but if the producer can guarantee the ordering when producing, it's a perfectly reasonable representation.

Surely there's a glossary for this sort of thing somewhere? please comment!

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Broader information in #200